I think Doppelganger is an alter, and his position in our headspace is probably one you wouldn’t guess. He’s mostly the caretaker for the troublemakers, namely Jag. He has bipolar and drinks too much caffeine, and he isn’t fond of children. He likes to scare them. He’s also an artist.
He’s a fiend and despite his frightening looks and attitude, he’s a good person. He hasn’t been around for a long time, but maybe he’s just busy look for/after Jag.