Guiltythree
His name, his true name, was taken from him. Not just fated, not just the bond that made him Nephis's slave, but all that entailed. Being fated was a curse and a gift, after all. So no more Uncle Sunny, maybe no more Sunless either for that matter. Rain won't remember him either. No one will. There goes his home, his job, his everything. Now, it makes sense why Cassie was so sad.
It could be a prank, still possible. It could also be that she thinks he is dead because the shadow bond broke and so the person in front of her is impersonating Sunny. It could also be that her saint transformation burns her humanity so much that she temporarily forgets people she knows. It could also be that because she was connected to his soul the thieving bird yanked out the memory of him out of it, so only she forgot him. The nearly worst case scenario is that because his fate was connected to his true name in a way, only everyone who knew it forgot him. that would be the full cohort but not Rain. The nearly-nearly worst case scenario would be that everyone forgot him so he has to make new relationships. The worst case scenario would be that he somehow became like Oblivion and everyone not only forgot him but will continue to do so the moment he is away.