Warnings/Disclaimer: Alternate Bleach Universe. Let me clarify that: Rukia is still a shinigami sent to Karakura town, Soul Society and Hueco Mundo still exist, Aizen is still the bastardly baddie, and Ichigo is still the main character. HOWEVER, instead of Ichigo becoming a shinigami, he's already a Hollow. This was actually my most popular multi-chapter story and I've very happy to put it back up again, as I'm sure many of you are as well :D Thank you all!
(p.s. I own nothing but this (apparently popular) idea (see Edit). Other warnings include mild OOCness, Yuzu can see the "supernatural" (kinda essential for the story), and maybe some language in later chapters.)
Edit:: It's been years and I forgot the title of the fic and thought I had lost it no matter how hard I looked! (it was in my favorites and I had reviewed, but I have over 1500 favorites so yeah, it was lost and couldn't find it xP) The Humane Hollow by Nate Grey. THAT is the story that inspired THIS! I finally found it again accidentally while looking for another! Our plots are different, but I first got my idea of a real Hollow Ichigo from Nate Grey. Credited and given permission to continue my story :)
Our Hollow Brother
Chapter 1 – Protective
"Ichi-nii! Where are you? It's time for bed!" A young, light-brown haired girl named Yuzu called out quietly into the darkness of her home's backyard as she stood in the open doorframe. She only had to wait a moment before a strange creature landed in the yard with a soft thump and slinked over to her.
Its large white form was covered in blood red "tattoos," and intimidating spikes adorned both broad shoulders. Wickedly clawed hands and feet, gold on black eyes glowing behind a toothily grinning red-striped mask, a thick reptilian tail, and an overall intimidating/fearsome presence that oozed strength should have given the young human girl a rightful fright. That is, if this creature called a Hollow and named Kurosaki Ichigo, wasn't Yuzu's deceased older brother.
The small brunette giggled when her older sibling purred echoingly and carefully nuzzled her cheek with the teeth on his mask before he began audibly sniffing over her, searching for any injuries she might've possibly acquired when he had been briefly away to kill another Hollow that had come too close to their home for his liking. He chirruped concernedly when he came across a tiny, shiny burn on her left thumb. The Hollow tilted his head up at her in question, crouched low in front of her, she only being slightly higher because she stood on the step.
"I'm okay, Ichi-nii! It doesn't even hurt!" Yuzu reassured confidently with a cheerful smile, loosely clasping her hands in front of her.
Ichigo's eyes shone of humored exasperation and the supernatural creature held out a hand to her, palm up and fingers inviting.
"But it's okay, Ichi-nii!" The eleven year old girl persisted, hiding her "wounded" hand with her other, not wanting to be a bother when the burn could just heal slowly in its own time.
The white hand held out to her flexed, insisting that she place her own hand within it. Yuzu sighed fondly at her brother's protective nature and placed her hand in Ichigo's. He immediately gently turned it over so the palm was facing up and the Hollow meticulously inspected the petite, vastly different hand more closely, searching for any other burns. Once satisfied that there were none, Ichigo turned his attention to the small welt on Yuzu's thumb. His mask's teeth separated and his rough, catlike blue tongue gently licked over the burn once. The wound fizzled painlessly before instantly healing and vanishing without a mark left behind.
Despite her protests that it hadn't hurt, Yuzu still looked relieved to be no longer hindered by the minor injury. Ichigo's dark eyes flashed knowingly at her and he briefly pressed his mask against her cheek again, reminiscent of a familial kiss. The eleven year old stepped back inside their house and the Hollow followed, claws clicking quietly on the linoleum floor and his long, thick tail weaving slowly behind him as to not knock over any furniture. Yuzu closed and locked the door behind him, flicking off the light switches before following him further inside.
The Hollow crawled on all fours through the house (because his tall height would otherwise his head would hit the ceiling), expertly weaving through furniture despite his wide, spiked shouldered size. Ichigo's tail gently wrapped twice around Yuzu's slight waist, lifting her onto the Hollow's back so that the elder brother could carry her up the stairs to her and her twin sister's room. The brunette giggled as he did this, seating herself comfortably right above his tail's base and a warbled purr drifted back to her ears when she played with his long mane of unruly orange hair.
Ichigo climbed up the stairs with fluid ease, slinking down the short hallway and nosing the cracked bedroom door fully open. Yuzu's dark haired fraternal twin sister, Karin, was already inside, sitting up on her own bed and reading a manga. She looked up briefly when her Hollow brother came in with Yuzu atop him. The raven haired girl rolled her dark eyes at the common sight and put her book on the bedside table before laying down and getting settled under her blanket.
Ichigo's tail lifted Yuzu off his back and gently settled the small girl in her bed. As soon as she lay down, he snagged the edge of her blanket with his claw tips and draped the comforter over her, tucking his baby sister in carefully with another gentle loving nuzzle to her forehead. Once assured that his youngest sister was well snuggled into her bed, he turned and inspected his other younger sister.
Karin rolled her eyes again as Ichigo also sniffed her over for possible injuries. She was confident that he had found nothing wrong when one clawed finger suddenly tapped her knee and the eleven year old hissed in pain, cursing as she hence revealed herself. Her brother hummed knowingly and gestured for her to let him see her knees.
She grudgingly did as he'd "asked", knowing he wouldn't leave her alone to sleep until he had healed her. Once she had pulled down her blanket and bent her legs, Ichigo sniffed critically over her scraped knees before rasping his blue tongue across the wounds soothingly. Like Yuzu's small burn, the raw wounds instantly healed with a painless bubbling (not dissimilar to what alcohol disinfectant would do) and faded away to only become a memory thanks to his healing properties.
Karin held in a sigh of relief as the mild pain no longer assaulted her but then squeaked embarrassedly when Ichigo unexpectedly hooked her right arm into the air and inspected her elbow, checking to see if she had acquired any scrapes there as well. Once confirming another wound's presence, he healed it as well as the one on her left elbow and on those on both palms. She'd practiced skating when he'd been gone again. Sniffing over her one last time, the Hollow deemed her injury free and tucked her in comfortably too. Ichigo chuckled throatily when the dark haired twin swatted at him for nuzzling her.
"Ichi-nii!" Karin protested when the Hollow persisted in nuzzling her until she pushed at his red striped mask with both her small hands and scowled at him. "Quit it." She demanded, fighting to keep a smile off her face to save her tough girl image when her older brother purred like a playful kitten.
He withdrew obediently, going back across the room to nuzzle Yuzu once more in his own version of a good night kiss for the both of them. Afterwards, he slinked over to the door and flicked off the light, retreating back to the center of the room and curling up on the carpet between Yuzu's and Karin's bed.
He began to purr, a soothing lullaby sound that, true to its purpose, encouraged the girls into sleep. Yuzu rolled over onto her stomach and dangled her hand off the bed to tangle her fingers in Ichigo's mane and she fell asleep that way, comforted by the fact that her older brother was close by. Karin curled up on her side, facing her siblings (alive and not) before she also easily fell into a peaceful slumber.
Ichigo continued to purr, knowing the sound helped his sisters in their sleep, but didn't sleep himself. He couldn't rest because of the other Hollow he had devoured would attempt to take over and he would never allow that to happen. He also needed to stay awake just in case any other Hollow showed up too near their house and he would have to go slaughter it to protect his beloved baby sisters.
Karin grumbled when she was coaxed into alertness by something smooth snuffling in her hair. She swatted lazily at it and it retreated so she let her hand drop. It soon came back, however, with renewed vigor and snuffled loudly right above her ear. Karin growled in annoyance and pushed weakly at the offender. Her eyes cracked opened into aggravated slits a few seconds later to see Ichigo's gold/black eyes looking mischievously down at her from behind his grinning mask.
The girl groaned and rolled over; attempting to return to sleep but her brother was having none of it. She felt a clawed hand wrap around her ankle but before she could sit up or protest, she was pulled up into the air and hung upside down above her bed. "Ichi-nii!" Karin bellowed in protest, arms and one leg flailing, trying to hit the Hollow holding her upside down in the air by her ankle. "I'm up! Let me down now!" She demanded, voice cracking from unconscious disuse.
Ichigo purred triumphantly and set her back down on the bed with stressed gentleness, ducking to avoid a socked foot to the face. Karin was irked at how his mask made him look like he was grinning insanely at her as he most likely was also underneath it. She grumbled at him as she got out of bed. Pleased, Ichigo left the room, closing the door behind him with his tail to creep stealthily downstairs and into the kitchen where Yuzu was already up and making breakfast.
The brunette squealed when she was suddenly scooped up into a pair of strong white and red tattooed arms and cradled like a baby with Ichigo cooing softly at her. "Ichi-nii!" Yuzu cried, giggling at the nuzzling she received- her brother's affectionate "good morning" routine. Ichigo set her back down on her feet before the stove before calling up the stairs.
"Karin! Get up!" His rough, warbled voice sounded like he was weirdly speaking from some air bubble underwater. He usually didn't talk because of how… unnatural it sounded, preferring to use actions to express himself rather than words but he spoke when wanted or needed to.
"I am up!" Karin groused back as she stomped down the stairs and plopped in a dining room chair. She swatted at Ichigo when he nuzzled her good morning and swore when she saw that her shoelaces were tangled in knots.
Ichigo snickered quietly at her and gently guided her sleep-clumsy fingers away from her red tennis shoes and deftly unknotted the laces, leaving Karin to pull them on and tie them up. The black haired girl mumbled thanks to both Ichigo and Yuzu, who had just placed breakfast on the table for the two of them to eat. Ichigo didn't eat with them, couldn't eat at all, but he stayed at the table anyways, softly asking in his echoing voice what they were going to be doing today in school.
Karin mumbled something unintelligible but Yuzu replied whole-heartedly. "We're reading poetry today, Ichi-nii!" She announced cheerfully. Ichigo leaned towards her interestedly, silently urging her to continue. Yuzu nodded while eating, pausing to chew. "I was thinking about asking my teacher if I could borrow a book so we can read it together!" The brunette chirped pleasantly.
Her Hollow brother purred approvingly. "That's a great idea, Yuzu. I'd love to hear you read some poetry."
Yuzu beamed at his praise and finished eating, taking her and Karin's (who had also finished) plates to the sink and washed them right quick before she put on her shoes and Ichigo handed them their school bags. He fussed over them, making sure they looked absolutely decent and then made sure they both had their house keys and all materials for the day before ushering them out, locking and closing the door behind him. He nudged them along the road towards their school, him following along beside them, walking on all fours again.
Yuzu suddenly remembered that she had a quiz today and so Ichigo began quizzing her with questions he had learned from helping her with her homework. She answered each one correctly and beamed when he confidently assured her that she'd ace the quiz. Karin rolled her eyes fondly but otherwise had no other comment.
They made it to school without much else happening and Ichigo bid them an affectionate goodbye, wishing them a good day. The Hollow watched them walk inside the school building before he turned and opened a gargantua to head into Hueco Mundo. He was only a few more Hollow away from becoming a Gillian but he needed a plan to go straight into Adjuchas so that he wouldn't have to leave Karin and Yuzu alone for longer than a day. He needed to become stronger to more easily defeat any Hollow to protect his remaining family, but he also needed to be small enough that he could still reside in the human world without any shinigami noticing.
Ichigo had it all planned out to evolve immediately to a Gillian and then straight to an Adjuchas in one day- or essentially a few hours because he needed to be back home in time for when his sisters got out of school. He always picked them up after school and he wasn't stopping his tradition today.
If a Gillian needed to eat a lot of other Gillian to evolve, then why couldn't he just eat one or two Adjuchas to speed up the process? But he knew he wouldn't be able to eat and Adjuchas in that huge lumbering Gillian form so he had to prepare in his normal form. Which it would be very difficult to render one, then two Adjuchas unconscious for him to eat while in Gillian form while he was still a relatively weak (in comparison) normal Hollow. It had taken a lot of preparation- studying and scouting the Menos Forest for the weakest Adjuchas, but he was now ready to execute his plan.
He was poised high up in a tree in the Menos Forest above an unconscious rather powerful normal Hollow (much like himself though he was still decidedly stronger than the bait) on the edge of the weak Adjuchas' territory, waiting and hoping for one to show up soon to devour the helpless bait. Ichigo wasn't stupid. He knew that though he was incredibly strong, there was no way he could just single-handedly battle, defeat and devour any Adjuchas so he was playing it safe and preying upon the weakest ones there were in a secret ambush. They still contained the number of Hollows he needed, so he wasn't bothered by their weak reiatsu.
He didn't have too long to wait until a birdlike Adjuchas appeared in the distance, obviously having "scented" the leaking reiatsu from the unconscious bait Hollow. It circled above the unconscious Hollow a few times, looking around suspiciously for a trap but Ichigo was higher up than it and it couldn't even sense Ichigo- he'd mastered the talent of hiding his reiatsu a long time ago in order to stay hidden from shinigami and remain with his sisters. He waited patiently until the Adjuchas decided it was safe enough and spiral downed to eat its scavenger meal. Ichigo's muscles tensed in preparation when the birdlike Hollow dropped its guard just enough to drop its head and expose its vulnerable neck to devour the unconscious Hollow.
Ichigo launched powerfully off the branch he'd been perched on, hurtling down towards the more powerful Hollow as fast as he could move, still completely unnoticed. He twisted silently in the air a few mere feet above the birdlike Hollow so that his tail was whipped around in a blur of barely visible speed. The thick muscle viciously cracked against the back of the Adjuchas' neck, Ichigo not daring to hold back an ounce of strength because of their different power levels. It paid off that he didn't because the Adjuchas immediately slumped onto the ground, its spinal cord snapped and its whole body paralyzed and silent but thankfully still alive for the next part of Ichigo's plan.
Ichigo grinned triumphantly beneath his mask. He quickly made sure that neither bait Hollow would, or could move, before swiftly jumping back up the tree branches, traveling up onto his high perch to wait for another target. This time the wait was considerably shorter, as more reiatsu was leaked out into the open air. Another, considerably weaker than the first, dog-like Adjuchas soon appeared, lumbering carelessly up to the meal.
Ichigo realized with a jolt that the dumb brute wasn't as cautious as the first had been and he had to use sonido down the tree with breakneck speed in order to reach the Adjuchas before it bit into either Hollow and completely ruined Ichigo's plan. The smaller, but slightly more powerful Hollow yelped canine-like when the larger red patterned Hollow slammed into its back just as its teeth had been centimeters from biting into the birdlike Adjuchas.
The sheer force of Ichigo's rapid, accelerated fall broke the Adjuchas' spine right in the middle, and a dust cloud erupted when the two collided into the sand. Ichigo himself was stunned briefly at the impact, but he was quick to clear the disorientation from his head and dig his claws into the back of the whimpering Adjuchas' neck, growling threateningly whenever the canine Hollow attempted to move its forelegs, its back end completely rendered useless from the collision.
Very pleased at how well his plan had worked, Ichigo quickly made sure the surrounding area was clear so he wouldn't be interrupted or attacked in the next stage of his plan. A sharp jab to the back of the only alert Hollow's head also rendered him unconscious and Ichigo was now tasked to drag the two unconscious Adjuchas (the bird appeared to have faded from being paralyzed) up into the tree so he could easily reach them while in Gillian form. Their weight was heavy and dense, compressed tightly with multiple other Hollows and their reiatsu but Ichigo was strong, young, and determined and he managed the load with only a slight struggle.
Once he had securely lodged the two Adjuchas into the tree branches, the orange haired Hollow dropped back down to the ground and finally bit into the regular Hollow's mask, knowing that this was the last boost he needed to evolve. Power instantly surged through him as he evolved into a Gillian, the regular Hollow dissipating as he did. His body felt painfully stretched into gargantuan size, lumbering and near pathetic in maneuverability. Everything was dim and he hated how sluggish his movements without any hands. Stepping closer to the tree, distantly glad that he had dragged the Adjuchas high enough to easily reach, Ichigo bit right into the tree, devouring both more powerful Hollow in one massive mouthful.
The effect was instantaneous. He evolved again, shrinking down to an even smaller size than his original form, looking much like his old self. The only differences was that he was slighter: more lithe and deadly, the spikes on his shoulder had disappeared, a sickle shaped barb appeared on his tail tip, and he now had red plated armor covering his entire body, excluding joints and areas where he needed to bend, the palms of his hands, and tail. His mask had also changed. The red stripes were a darker, deeper red, and the stripes angled more upwards to the top of his mask, which now formed a little more over his head like a skull rather than just a plate over his face. His long mane of unruly orange hair was still as untamable as always and red tattoos could still be seen between the armor, seemingly to connect the protective plates together.
He still had approximately an hour before he had to leave and pick up Karin and Yuzu so Ichigo began testing his new strengths and powers in this form so he wouldn't accidentally harm either of his sisters. Vigorous training on trees and other Hollows resulted in Ichigo concluding that he knew all of his Adjuchas powers and how to pull his movements just enough to feel like second nature and gentle enough to not even hurt a baby. Extremely pleased with himself, the Hollow Kurosaki opened a gargantua and stepped out of Hueco Mundo and back to the human world, effortlessly hiding his increased reiatsu as he did to continue remaining undiscovered.
Ichigo crouched comfortably outside the school gate, his new smaller barbed tail lazily swimming through the air behind him as he waited, subconsciously careful not to hit any passerby who couldn't see him. He only had to wait a few more minutes until the dismissing bell rang and the students were released.
Yuzu was soon seen walking towards him amidst the other children with a book hugged to her chest as Karin walked half a step behind her, the dark haired sister's small hands folded behind her red baseball capped head. Yuzu was the first to reach him and notice his transformation.
"Ichi-nii, what happened? You've changed?" She whispered so that no one would see or hear her apparently talking to nothing. She knew no one else but Karin could see their older brother and didn't want to be thought of crazy as Ichigo and Karin had cautioned her of.
Ichigo purred at her. "I'm fine, Yuzu. I just evolved and got stronger to better protect you both." He explained in the lamest terms possible, not wanting to overwhelm his more innocent baby sister with the details of how he had accomplished this transformation. It seemed to satisfy her and he ushered them alongside the sidewalk towards home.
Yuzu was happily chattering about her day to Ichigo, who was walking along on all fours to be her height. The light brunette was seemingly talking back to Karin who remained silent as if listening. The dark haired sister was actually observing her brother's new form. As usual, he looked perilously deadly, if not even more so now, but she had unwavering confidence that Ichigo would only do this to protect them better and she had no fears whatsoever that he would ever hurt them, even accidentally. She knew he would never forgive himself if he did.
Karin jumped at her name having been called and looked up, not having realized that she had stopped walking while she had been musing over her brother's transformation reasons. "Uh, sorry. Was thinking." She said simply in explanation, walking quickly again to join her siblings and walk beside Yuzu on the opposite side of Ichigo.
Yuzu began happily chattering again and Ichigo listened to her but kept shooting anxious looks towards Karin in concern for her momentary lapse. The trio reached home without any further incident and Yuzu immediately went into the kitchen to make an after-school snack for them before they started on their homework.
Ichigo lightly touched Karin's hand to gesture for her to follow him into the living room for a quick word. "Is something wrong?" He inquired quietly, his concern clearly heard even through his warbled voice. "I know I look rather… but you know I would never hurt either of you." He assured her, unable to come up with an appropriate term to suitably describe his fearsome appearance.
Karin nodded with a tiny, reassuring smile. "I know, Ichi-nii. Sorry for worrying you. I really was just thinking about how overprotective you are." She teased him with an eye roll and faux irritably crossing of her arms.
Ichigo scoffed at her but the sound was saturated with relief so it ruined the effect. Karin customarily swatted at him when he nuzzled her, the motion oozing stressed gentleness and thankful affection. They were about to go join Yuzu in the kitchen when Ichigo suddenly stiffened and his head whipped around towards the door. He looked back around to Karin after a moment of silence.
"I'll be right back." The Hollow said quickly before he slinked over to the door, opened it and jumped out, the door swinging shut behind him from the low air pressure resulting from his abrupt exit.
Karin nodded even though her older brother was already gone, knowing the reason behind his abrupt exit was that another Hollow had showed up too near the house and he had gone to kill it before it could get any closer to their home and hence her and Yuzu. She walked into the kitchen to tell Yuzu that Ichigo would be right back and to eat her snack.
A raven haired shinigami blocked an attack from the Hollow that she was fighting and swung her zanpakatou up to force it away from her, shunpoing back a few steps to gain more room to swing her blade. She was just about to release her zanpakatou to end this drawn-out fight when a brilliant red Cero suddenly slammed into the Hollow before her. The darkly clothed "young" woman had to dive and roll to the side to avoid getting hit as well.
Since it was a rather powerful Hollow (hence the reason why she was still fighting and had been about to release her shikai), it survived the attack but just barely. Astonished, Kuchiki Rukia looked in the direction from where the powerful attack had originated and was shocked to see a rather small, lizard-like humanoid Hollow nearly completely covered in read armor hurtling towards the weakened Hollow and herself.
The shinigami had learned that Hollows ate other Hollows to evolve but she thought they only did so in Hueco Mundo. Disregarding the currently useless thought, Rukia prepared herself for anther fight but was startled when the lizard humanoid Hollow roughly blew past her without even a glance in her direction and pounced on the one that had previously been attacking her. She was fully prepared to see the new Hollow bite into it but was shocked when it did not. Instead, the newcomer merely tore the other Hollow's head clean off and blankly watched it dissipate without so much as even opening its jaws. Rukia had never seen a Hollow merely killing another and was immediately put on guard. Her Nii-sama had informed her that those rare few that did, were intelligent and hence exceedingly dangerous.
When the reptilian Hollow straightened to its full height of six feet something, she noticed something odd. It had no spiritual pressure. Her violet eyes widened. Impossible! How had a Hollow this obviously powerful learned how to hide it? Unless it naturally unconsciously did so or held a special cloaking ability? But even that seemed ludicrous. What would be the purpose of hiding its reiatsu when it was standing right in front of her?
Rukia prepared herself for an attack when it turned and faced her. Instead, the strange Hollow slowly blinked gold on black eyes at her and swiftly jumped away, its job apparently done, back in the direction it had come from. She stared baffled after it before her training kicked in and she gave chase. It was her job to purify all Hollows she came across and this was a Hollow, weird or not.
It suddenly changed course, going almost horizontal to its original direction, when it realized she was following it. It led her to a park and landed silently before turning around and facing her, waiting for her to join it. Rukia was perplexed. When the shinigami touched down a good safe distance away, it spoke before she could.
"I don't have time for this. Why are you following me?" When it spoke, its voice was the usual Hollow echo but it was definitely masculine with a curious tone, barely tinted with irritation. The fact that it could speak with actual proper grammar (rude of course, but all Hollows were) confirmed that this Hollow was exceptionally intelligent and Rukia was right to be wary of it.
Hand still clasped tight around her sealed zanpakatou, Rukia narrowed her eyes suspiciously at the red plated Hollow. "Why did you kill your brethren, Hollow?" She demanded, readying her stance for a possible attack.
"They're not my brethren." The Hollow snapped roughly, making Rukia automatically tense in preparation but still no attack was forthcoming. The Hollow's sickle-shaped tail swished irritably behind the tall humanoid reptilian Hollow. "It got too close anyways." The Hollow seemed distracted when he said those words, masked face tilted slightly to his right as if seeing something she could not.
Rukia's eyebrows rose. She hadn't been expecting that for an answer. "Too close to what?" She interrogated, not daring to relax from her wary stance but willing to provide pressure to get any answers in this sudden mystery.
The Hollow's eyes slid back towards her, and he was more guarded with his next answer. "You don't need to know what. Just know that if any get too close in this area, I will kill them." He stated, deliberately wary in giving her a direct location.
Rukia frowned at his answer. "Why? What's in this area that a Hollow would kill his own kind for?" The raven haired shinigami tried to tamper down her individual curiosity, instead trying to remain administrative in her questioning of this Hollow's motives.
"I won't tell you. I don't want them to be disturbed or brought into this. I have to go back, they're waiting for me." The reptilian Hollow spat, obvious distaste in his voice now (undoubtedly from her prodding), and before Rukia could say or do anything more, he disappeared in a buzz of sonido so swift that she couldn't follow where he went and with his seemingly nonexistent reiatsu, she couldn't track him.
Rukia remained frozen stiff in her stance for a solid four minutes, every trained sense that she had alert just in case of a surprise attack. By the fifth minute, she felt safe to assume that the Hollow had just left her alone and she exhaled silently as she finally allowed her tense muscles to unwind. He'd gotten away, but there was nothing more she could do on the matter when he was decidedly more powerful than she. But what was he? And who or what did he mean by "them"? The thought of more Hollows like him worried her. That had been no ordinary Hollow in any way possible. She sheathed her zanpakatou and headed towards Urahara Kisuke's Shop. Maybe he could possibly give her some answers to this strange Hollow's behavior and possibly the reasons behind them.
See Edit note at top of fic!
Both Yuzu and Karin know that their older brother is a Hollow and where he goes such as Hueco Mundo and off to kill other Hollow and such. They also know about shinigami cuz they can also see them and Ichigo tells them everything. Better they know about everything and what's happening than be left in the dark and hence in greater danger.
Their parents died when Ichigo just turned 13 and so he was left to care for his younger sisters, both 9 at that time. He took care of them, went to school for a little while longer, had a job, and everything. But he was in an accident when he turned 14 (Yuzu and Karin were 10) and died, then became a Hollow. So they've spent a year together with Ichigo as a Hollow, so he's now technically 15 while Yuzu and Karin are both 11, like in canon.
I'll be explaining more of all this in greater detail in later chapters so never fear! In the meantime, please review about the extended chapter! :D it was previously chapters 1 & 2 now put together, and I'm much happier with this as the first chapter ^^