Durarara!!

[Yukimura]

That's an interesting idea. I have been assuming that silent neck scar girl didn't have much of a personality and Seiji fell in love with the idea of her and was projecting his romantic ideals onto her. It seemed/I would like to believe she was more along for the ride because he seemed nice enough and she didn't know any better. It could be that while her body desperately seeks to reunite with her head her head has somehow been 'tricked' into accepting her current body and is satisfied with her state and is moving on with life.

There is most likely some sort of link between the pharmaceutical place and Seiji beyond just the fact that his sister works there and the hints we've gotten so far seem to point to Silent Celty being a product of whatever program the pharmaceutical place is working on. I wouldn't be surprised if Seiji was at least partially aware of the circumstances surrounding Silent Celty's current state and trying to keep her for himself.

On an unrelated note, domestic Celty succeeded exceptionally in pushing my moé buttons with her reactions to Shinra eating her food. So cute.