Disclaimer: Does this happen in the manga or anime? No. Hence, I do not own!
Rukia is arrogant.
In her mind, Ichigo has been hers ever since her zanpakto was thrust through her chest.
She was wrong.
Ichigo was Tatsuki's the moment the six-year old tomboy told him to stop crying and held out her hand. He was Chad's the moment the Mexican protected his back that first time. He was Orihime's when she came up, blushing, to introduce herself before offering him some of her lunch. He was Uryuu's since the Quincy showed up to help rescue the Shinigami without being asked. Keigo's since he went running for help, Mizuiro's since he claimed to be good at making excuses. Chizuru since she almost sacrificed her life saving his, saying his death would hurt Orihime.
Rukia takes Ichigo's virginity. He is sixteen; she is older, much older. He is a virgin, she is not, has not been for a long time, since she lived in Rukongai, when she and Renji shared body warmth and were desperate for food, since before she met Kaien and then imagined that everyone else was him.
They were not in Sereitei. Instead, they were at Ichigo's house. His father is at a medical conference (practically thrown onto the plane by his two oldest children as Yuzu tearfully waved) while the girls are at camp.
Rukia smiles at the small bottle Mayuri gave her, before putting it in water. Ichigo needed stronger ties to Soul Society, so that they could trust him not to betray them.
It takes three days until they do it. They do it in Ichigo's room, on his bed. Ichigo is almost drunk while Rukia pretends to be, alcohol on her breath (she can drink Renji under the table if she wants to). She drags Ichigo's face to hers, her tongue twisting with his. They go into his room, and then she's lying on the bed, loosening her robe. Her robe slips from her shoulder, bunching at her waist, and then opening so her legs are bare. Ichigo gets drawn down to her.
And then Ichigo is inside her. Rukia bites her lip, gasping at the sudden burst of spiritual energy slamming into her, surrounding her.
She finds it hard to breathe. She knows very well that she isn't the strongest of shinigami, never has been. Since learning that her brother blocked all attempts for her to be a seated shinigami she stopped trying, becoming complacent.
The reiatsu was unlike anything she'd ever felt before, and awakens an unknown hunger inside her.
Ichigo lies inside her, panting, resting his head against her shoulder. He seems unprepared, lost, with no idea what to do, what to hold, arms resting on the bed or either side of her head. Rukia slams her hips up, driving him further into her, she groans at the motion before repeating it. Soon, she is coming; feeding off the high his reiatsu gives her, as well as his reiatsu. She smiles at him before allowing herself to drift off, unnoticing how Ichigo stares at her with a strange expression, having lost his arousal.
He looks too much like Kaien, almost his younger twin without the hair. The second time, she is on her hands and knees on the bed, having guided Ichigo into her body before pushing back. She comes, not paying attention to the name that slips from her lips.
She grows to prefer being taken from behind, as she always did, it's her favorite position since it allows her to pretend. The few times they do it missionary, she closes her eyes.
When she returns to Soul Society, she smirks as she notices Renji eye the few bruises on her arms (Mayuri's drugs helped Ichigo's libido). She pushes him against the wall, leaning up to meet his mouth with her own. Seconds later they are naked and on the floor of Renji's office, Renji tasting Ichigo's reiatsu on her skin.
Urahara are Yoruichi are both ex-shinigami captains. They know what Rukia is doing, what she's done, how she feeds off of Ichigo's reiatsu, getting stronger each time they share their bodies.
Ichigo knows too, after the first time he found himself unable to defeat a mid-level Hollow, and then the next day he almost falls to another. It takes a week before he feels prepared to fight again; after she leaves he's recovered after one day of rest.
When they learn, Ichigo's friends are furious (Ichigo is in Urahara's shop, sick with a fever due to reiatsu exhaustion).
Uryuu wonders if perhaps deciding against killing shinigami is not such a good idea. Orihime looks at the hair clip holding Tsubaki and wonders how long it would take her to master her offensive abilities. Tatsuki decides to do more practice. Keigo glances at the flyers for self-defense classes he'd grabbed when he learned monsters existed. Mizuiro remembered why he disliked females and contemplates if he should carry weapons to defend himself. Chad contemplates the idea that fighting to protect Ichigo would also mean getting revenge on those who hurt him. Chizuru decides that Rukia is not attractive, in fact, the shinigami is rather ugly.
Rukia comes back, her reiatsu holding traces of Renji in it. Orihime approaches Ryuken Ishida. A month later, the shinigami learn that none of them can enter Karakura town. Ryuken holds a cigarette to his lips, gazing at the barely visible shield.
Knowing the shield keeps out all outsiders is well worth the fact that he had to work with shinigami and vizards to create it.
That first night, the first night the kidos Rukia used fall apart. Ichigo awakens, finding his sheets bunched around him, dampened with his release, images of his Hollow fading from his gaze.
For the first time, he doesn't feel empty.
Shiro, Ichigo's Hollow, shows up a week later. He grins at them as he is introduced as a transfer student (one of Ichigo's classmates managed to win the class-wide bet for when the new student who suspiciously knows Ichigo would arrive, and another won a separate bet in that it would be just one guy).
Shiro takes Ichigo in the boy's locker room, teeth digging into Ichigo's shoulder, making sure to look straight at Ichigo and say his name when he came (on a side note, the classmates who won the bet that Ichigo was gay pooled their prize money together to go to the onsen for a week, their homeroom teacher doesn't mind the celebration party they hold since she's invited along to supervise).
Ichigo's friends try not to wonder how many times Ichigo's gaze goes blank his Hollow is molesting him in their inner world.
Tatsuki is the first that that is invited to their bed. She is Ichigo's oldest friend, his first best friend. She was never in love with Ichigo, apart from a brief crush on him when they were 5 and they accidentally kissed and then later when she was 13 and saw him beat up a bully, but that was more liking his smile and admiring his strength.
She wonders how much the invitation is an invitation and how much it is an attempt for Ichigo to exorcise Rukia. Ichigo always fucks her in a position where he can see her face (such as with her with her legs around his waist as she felt Shiro against her back, fucking her in a way that no one had since the previous year when she was horny and on her period).
She gets to watch the two of them have sex afterwards. Watching the pale skin move against tanned skin drove her to orgasm alone, causing her to come all over the sheets.
Orihime is the next to be invited to their bed. Orihime, who loved Ichigo because he was strong and untouchable. She hasn't been innocent since she was 14, and had her first boyfriend. Her first boyfriend who insisted on having sex in the dark, and she later learned that it wasn't always him in the room with her. Her boyfriend laughed, while pocketing the yen he'd received, stating that she was just a whore, it was business.
Tatsuki, who just extended a hand of friendship, beats him up before promising to protect Orihime. After meeting Tatsuki it becomes safer for Orihime to walk home.
Orihime loved Ichigo because, somehow, she knew that he would not be interested in her. Tatsuki is the one to urge her to accept the offer.
Ichigo allows her to top him, straddling him as Shiro was a firm wall against her back, sliding in and out of her ass as Ichigo fucked her. Orihime giggles as she watches the two slide against each other.
Tatsuki was right, it was hot.
Rukia ends up coming back once, Renji and other shinigamis' reiatsu on her skin. Shiro sends her flying with a glare, balls deep in Ichigo who didn't even notice her entrance (strangely, after that happened, Ichigo's window got damaged enough that it had to be replaced, which it was by a window that didn't open and was made of shatter-proof glass). The shield gets put up shortly later.
Orihime and Tatsuki announce to their friends that they are dating, no one bats an eye, no one was surprised (though Chizuru did cry before offering to comfort Orihime when they broke up and promptly received a hit for her offer).
The two girls offer to share an apartment with Ichigo for college since their colleges are neighbors. In the end, they are neighbors.
And only those four know how many times any of them slip next door.
None of Ichigo's friends mind Shiro's presence. He'll never leave Ichigo; and even though he would claim otherwise he'd rather die than hurt him (unless it's during sex, in which case all bets were off).