[ESSAY] Rabi and Bookman
Rabi's Bookman formed an attachment. It comes as no surprise that's what happened, with Rabi being an Akuma and all.
However, afterward, Rabi's Bookman was repulsed by himself--regardless of other reasons to bring Rabi back. After all, a machine would surely be better as a Bookman than a human, undying and not easily destroyed.
Bookman waited, watching Rabi as a wild Akuma, tearing people apart with unabashed glee--his hatred of humanity coming out in a horrible way. When Cross caught him, Bookman practically had Cross whitewash Rabi's personality. Limiting it and tying the boy to Bookman as a dependent, fearful that Rabi would return to that wild creature.
Rabi struggled with the changes, but Bookman offered no support for the still young boy. The relationship between the apprentice and master stiffened and became cold. Bookman forced Rabi into a servant-like role, only talking to him as a person when they were playing the part of being normal travelers.
When they reached the Order, Bookman willingly outed Rabi as an Akuma, knowing he would be found out eventually anyway. To convince people Rabi was of no threat, Bookman added to his orders that Rabi would serve all members of the Order like he served Bookman. It wasn't long before Rabi became sort of the 'pet Akuma' of the Order, more like a piece of furniture than a person.
Sure Lenalee was nice to him, and Kanda wouldn't have anything to do with him, but no one really treated him like a real person until his Allen came along and insisted upon it.