The first time she doesn't even think about it. The enemy is before her, its sneering Hollow mask a chilling contrast to his human face and he introduces himself, practically bleeding arrogance. And in the split-second moment that follows, Rukia knows she cannot reply with the name Kuchiki. It is one thing to give it to friends and allies (however unpleasant), or to use it as an alias at Ichigo's school, where it means nothing. But it is another entirely to utter it in an Arrancar's presence. It feels strangely like deceit.
It is not like she asked for it. She hadn't wanted it at the time, still isn't really comfortable with it now. Despite its prestige, the Noble House of Kuchiki has done her no favors. It is only her respect for Nii-sama that she does not cast the name Kuchiki off entirely.
She's thought about it, several times over the years. It leaves a bitter taste in her mouth every new person she introduces herself to, shame drilling a hole in her heart. She knows it's not right that she uses this name, but she doesn't know what to do, which is the best way to do right by Nii-sama. He did adopt her, after all, allow her to live in his ancestral home, and provide for her in all the ways real families were supposed to, even if the real reason for her adoption still makes her a little sick to her stomach. Rukia could never fathom his mind, does not know whether he would prefer her to leave his name out of her treachery or be insulted that she rejected his gift.
So she compromises, walks the thin line between two stark options, giving it to those she trusts only. She does not know what else to do, and for the millionth time she ungratefully wishes Kuchiki Byakuya would have just passed her over.
The Kuchiki name is a part of her, undeniably, but in two years time, she will not be lying when she tells Ichigo that it does not belong to her. She's had it now for more than forty years, but it has never been hers. It probably never will be.
But she stands before the enemy now, one of Aizen's creations and she must reply. Rukia is a traitor and an exile, a liar and a collaborator with some of the Soul Society's greatest criminals, and she knows she cannot attach the Kuchiki name to that even more than she already has.
So she does not even stop to think, and uses Ichigo's name instead.
It becomes a habit after that, a convenient way to distance herself from one of the highest ranked noble families in the Soul Society, to shield them from the dishonor of being associated with a defector. Most of her battles are fought alone, so it's not until months later, in Hueco Mundo, that Ichigo overhears her.
"Did you introduce yourself as Kurosaki Rukia, before?" he asks her later, after they have brought down Yami, as she uses kidou to heal the worst of an unconscious Sado's wounds until Inoue and Ishida return from the roof.
In any other circumstances Rukia might have told him to drop it, but her arm is badly broken, crushed from Yami's grip, and Rukia's just so relieved that they're all alive and too worried about the defeated look in Ichigo's eyes to deny him.
"I can hardly use my name," she replies, moving one glowing hand down to Sado's busted up ribs.
Ichigo blinks in confusion, crossing his arms over his chest. "Huh? Why?"
Sometimes she forgets how little he knows about their world. No. Her world. Not his. The idea of a shinigami not knowing of the Kuchiki Clan is laughable, but then again, they are not shinigami. Not anymore.
"I am a deserter," she finds herself saying bluntly, looking up to meet his eyes calmly. "I will not drag the Kuchiki name through the mud with me if I can help it."
Ichigo scowls at this, obviously not happy with the meaning behind her words, but he doesn't call her on it. Instead, he raises an eyebrow testily.
"Right..." he says slowly. "So, what, my name is totally fine to use then?"
Rukia suppresses an absurd smile. "Ichigo," she says candidly, moving to a new injury on Sado's chest. "You are a ridiculously over-powered human with no control over your spiritual power, working closely with Soul Society criminals. To be honest, I am not sure how much worse of a reputation your name can get."
But he doesn't complain too much, or even ask her to stop doing it. Rukia thinks he's actually kind of flattered.