NOTES (for those who know the series, spoilers for those who don't)
Rabbit has taken on the persona of Lavi as his true self at ES, as that's all that feels natural to him. He fights his heartless Bookman side, believing that somehow he really does have emotions, namely compassion, inside of him and isn't some sort of emotionless freak. The first sign of the Bookman happened when people were going insane over the voices in his head and somehow he managed to stay calm, sorting out all the voices.
As he continues on and the curiosity about the true nature of the Sphere eats at him, the Bookman side does surface every so often, becoming aware of how controlled his emotions are and his general lack of compassion. He's taken Eyepatch's advice and accepted things as they are, hoping that Murphy's law will kick in and bring him something from the past. It still bothers him greatly, but he bottles it up inside.
The longer Rabbit stays here, the more people he slowly becomes attached to, accepting them as family. Unfortunately that family began to disappear, all within a two-month period, causing Rabbit to become more and more cold on the inside while maintaining the Lavi-like persona on the outside. It just feels better that way. Only his housemates, Juliet and Black, have seen his true side, the emotionless Bookman.