I might make a more in depth post about this later, but there’s something about Allen and Mana’s ways of grieving that stick out to me. They’re so similar, but I think there’s one key difference:
Mana forgets, Allen doesn’t.
When confronted with guilt over hurting a loved one (regardless of circumstances) and the grief of losing them, Mana couldn’t stand to look at himself to the point he literally changed his own face. He spent years searching for the very brother he killed. He hated himself so much he became someone else (Adam). It wasn’t it his fault (Adam’s), it was his fault (Mana’s)
Allen is similar, but ends up swinging in the opposite direction. When he’s at his lowest, he couldn’t cope with it either, becoming catatonic (is that the right word?). After he comes out of that state, it’s because he needs to keep a promise that he (Allen) made to Mana. And like Mana he’s acting like a different person! But he’s still Allen and he carries the guilt of turning Mana into an Akuma directly. He can’t change his face. Everything about his appearance is a constant reminder. It was his fault, and he’s going to spend everyday atoning for it.