I absolutely absolutely know what you mean. With my twin that has schizophrenia, there are entire months where I'm convinced he's never coming back. That he's permanently gone this time. Then when I give up, there will be just one exclusive random day where he'll be a normal functioning person.

It sounds harsh, but when he's in that illness 'phase', the person seems more like a demon-possessed creature and not even like much of a person. When you hear the things and intonations that the person will say in conversations to themselves in particular.. that's what sounds the least person-like.