What had started as a rather petty prank on his father quickly spiraled out of control and took a life of it's own.
Itachi decided in a fit of spite to follow in the footsteps of the Red Habenaro, Kushina Uzumaki, after he saw his father hurt his brother. It wasn't that he physically hurt Sasuke...Itachi would have mauled him if he had, but the fact was that Fugaku wanted to follow Itachi on a mission around the same time Sasuke entered the Shinobi Academy for his first day.
He nipped that in the bud rather fast, but the truth was that Sasuke had known his 'father' had forgotten about the fact he had a second son.
And that pissed Itachi off immensely.
(Un?) Fortunately, Itachi knew exactly what he could do to put his father in the doghouse for the immediate future and royally piss off the increasingly irritating Elders at the same time.
He could create an illegitimate Uchiha...one that was directly related to Fugaku. If there was one thing the Uchiha hated, it was when someone claimed the bloodline but wasn't born to an acceptable woman to the clan. In other words, a bastard child.
His mother Mikoto, was a rather jealous woman who would be beyond furious if she found out Fugaku had cheated on her after they were married. So much so that she would make his life a living hell on earth.
So Itachi plotted against his father in a way his mother's genin teammate Kushina would have approved of. He did his research, waited until he found the perfect mission that would take him outside the borders of Hi no Kuni, well out of the way most Uchiha would dare to go...and upon his return created a shadow clone of himself as he used a henge to disguise himself as a female.
Going into Konoha as a boy could cause problems...the odds of him slipping and revealing it had been a prank were too high. A girl, on the other hand, was not his default state and made it easier to keep the two personalities seperate.
And so Yoruichi Uchiha was born. To be fair, this also doubled as an assignment he had put off because of how noticeable he was as Itachi.
Izumo and Kotetsu, collectively known as the Gate Guard duo because of how often they landed the duty (primarily because they kept causing pranks and angering the watch commander), were bored out of their minds. So the fact Itachi was returning with a rather attractive girl about the same age with long raven black hair and an already considerable developing bust had them hoping for some entertainment.
Maybe Itachi had found a girlfriend?
That thought came to a grinding halt the second the girl had taken one look at them, flashed a grin that was pure sin...and activated what could only be a fully-developed Sharingan. Their minds did a four-second standstill.
Itachi smirked at them.
"Itachi-san, who is this?"
"I found someone interesting while I was away on the outskirts of Kiri," he said, amusement clear in his voice. "Meet Yoruichi Uchiha... my half sister, according to her."
Stone Silence. That was the only way to describe it.
"My mom met a Konoha nin shortly before the purges started by the name of Fugaku. I've been on the run for months since these dumb eyes activated," she explained.
By the time 'Yoruichi' had her bloodline confirmed at the hospital, despite the fact the only thing she owned were the clothes on her back because they villagers had burned down her home on the outskirts of Kiri, word had already spread to the Uchiha clan about the new girl claiming to be Fugaku's illegitimate daughter.
Mikoto, when she found the girl was Fugaku's child by blood, was pissed beyond reason. Mainly because due to the girl's age (according to the Iryonin) she had to have been conceived roughly a year after they were married. And no matter how much he denied having cheated on her, the test was impossible to fake.
Yoruichi was his daughter.
Itachi would laugh silently at the misery he put his father through. And somehow, he just knew Kushina was laughing with him in the afterlife.
She definitely would have seen the humor in the way Itachi was making his father's life hell and getting away with it. As it was, Itachi got to enjoy making Fugaku as miserable as possible while giving those damn Elders coronaries because of a Sharingan and a female outside their control.
Oh yes...revenge was sweet.
Yoruichi was in the girl's side of the hot spring, using a hole that the females knew was there but couldn't get rid of. She was giving the guys a taste of their own medicine by peeping on them for a change, while giving critics of their 'assets' for the other women.
It horrified some of the more prudish older ladies, but the kunoichi were having a lot of fun taking turns with her and giving their own assessments of the guys in overly loud voices. Anko was particularly vocal.
"That was fun," grinned Anko. They had been forced to leave the hot springs for a while because the men complained...and an off-duty ANBU had threatened to turn them in to the police if they didn't knock it off.
Fugaku hated his 'daughter', and Yoruichi wanted nothing to do with the clan (outside of Sasuke, Itachi or rarely Shisui) as long as Fugaku denied he had a fling with her 'mother'.
If things kept going like this, Itachi would have to end the clan...though not before he revealed the fact that Yoruichi was just him in a henge and women's clothing sent to make his father's life a living hell.
Mikoto refused to even let her husband touch her, because Itachi took perverse glee in reminding her about the 'bastard child' by bringing up Yoruichi at the worst possible time. Or by getting Sasuke to do it for him.
The look on Fugaku's face was almost as entertaining as the fact that Itachi knew his mother went to the lone survivor of her genin team for comfort... Hitomi Hyuuga. Wife to the current head of the Hyuuga clan, Hiashi.
Hiashi had no problems with the fact his wife and the wife of the Uchiha clan head got up to some pretty questionable activities in the bedroom. So long as they remained discreet and avoided the Hyuuga clan house, it wasn't his problem.
Besides, the dislike between the two ran deep, so anything that made Fugaku more miserable than usual was fine in his book. Itachi briefly wondered if he should clue the Hyuuga in to his prank...but the fact Yoruichi doubled as his ANBU personal kept him from sharing it with them.
He was sure Hiashi would have a good laugh at Fugaku's expense if he knew the truth.
Back to Anko and Yoruichi though.
"Oh man, I haven't laughed that hard in ages! Did you see the looks on their faces when they realized what was going on?" cackled Anko.
Yoruichi snickered evilly.
"Not our fault the boys have a long list of...short...comings," said Yoruichi innocently with a waggle of her eyebrows. Anko cackled in agreement.
"So what do you want to do now?"
"I'm thinking...dango. Because for some reason every time I try to train an annoying Uchiha shows up determined to steal any skills I have. Bastards."
Yoruichi found it impossible to continue what little training she had because the Uchiha clan was determined to steal her skills. And since she preferred not to use her Sharingan to copy the abilities of others when she could earn them herself, that meant she had to find new places to train where they couldn't find her.
At this point, she was a regular visitor at the hospital...and she was scheduled to take the iryonin exam so she could get more advanced scrolls.
Most shinobi avoided the hospital like it was the plague, so it made perfect sense for her to go there to learn how to be a medic to piss off the Uchiha, who thought such duties were beneath them. Why should a 'mighty' Uchiha spend time helping their comrades and saving their lives? Let alone waste their precious chakra on them?
As a consequence, Yoru was becoming quite the medic.
After eating dango with Anko, Yoru came across the unhappy form of Sasuke.
"What's eating you chibi?" she asked, ruffling his hair.
"Itachi-nii poked me again and said he would train with me next time," pouted Sasuke.
Yoru bent down.
"Tell you what. I could use some practice with shuriken and kunai myself, so why don't we train together and rub Itachi's face in it?"
"And Tou-san's?" asked Sasuke impishly.
Sasuke knew that Itachi was using their new big sister as a way to get their father to back off on him. And Itachi suggested Sasuke use their new sister as a way to get back at their father for ignoring him.
Sasuke was all too happy to help Itachi make their dad unhappy because their mother would tense up every time they mentioned Yoru-nee-chan. Their parents never explicitly said it, but the existence of his new nee-chan was always a sore point between them.
Especially since Yoru-nee-chan refused to listen to their dad because he kept denying she was his daughter.
"Really. Want a ride?" she asked with a grin, leaning down so Sasuke could scramble up her back. Sasuke did so as fast as a monkey, his grin wide enough to practically split his face in two.
Itachi rarely had time for him, but Yoru-nee-chan almost always trained with him. Plus it was funny to see his father's face whenever he mentioned his sister in front of his mother. It was almost like he became constipated.
Itachi showed up shortly before they finished for the night, and took Sasuke home.
"Aniki, why are you in your ANBU gear?"
"I'm going on a mission tonight. To the Land of Ogres. I'll be back in a week, but I wanted to say goodbye to you and Yoru-chan first," he told his little brother.
Actually he had to swap out with Yoru before he left. No need to risk the prank after all...though the fact he got to embarrass his ANBU commander earlier was pretty funny.
Itachi did come back a week later, and almost immediately after giving his report went to visit Yoru. He promptly switched with a shadow clone and slipped into his female persona.
He needed it after what he just brought back.
In payment for his bodyguard mission for the priestess that ruled the Land of Ogres, he was allowed to keep one jutsu from their private archives. Most would have gone after their flashy or powerful jutsu.
Itachi chose one that surprised his employer.
The private guards had a jutsu that, when used, would allow them to permanently take the form of someone else. It was so thorough that the first time it was used, an iryonin confirmed the transformation was so complete that the test subject could have become pregnant in that form.
The reason they were surprised was because outside of the private guard of the priestess, there wasn't much call for a jutsu that allowed a permanent transformation.
However Itachi did have a use for it. If what he feared became true, and the Uchiha clan went through with their plans of taking the Hokage seat by force, then he needed a way to disappear.
If he had to, then he would become Yoruichi permanently.
"Aniki! You're home!" said Sasuke with relief.
"Hello otouto. How was the Academy while I was gone?"
Sasuke's good mood vanished.
"Father's become angrier every time Yoru-nee-san is mentioned. He's all but forbidden her name while you were gone," said Sasuke.
"How bad is it?"
"He's had some of the older members collect me every day at the Academy, so I haven't been able to see her at all while you were away."
Itachi leaned down to eye level.
"Sasuke, I want you to listen to me very carefully. Father and the other Elders have been acting rather irrational of late, even more so since Yoru-chan came here. If there is ever a moment when you are coming back to the compound and you feel like something is wrong, I want you to head straight for Yoru's apartment and stay there."
"Something bad is coming. I don't know what it is, but I've noticed it's been coming faster and faster lately. I don't want to worry about my favorite otouto being hurt by whatever this is," said Itachi.
Sasuke saw how serious his brother was being, and nodded. If he ever came home and sensed something was wrong, he would head right to Yoruichi's apartment without hesitation. If Itachi was worried, then something really, really bad was about to happen.