Would there be any way to do it that's, um, not a real life mental illness?

Fairly certain that's the issue here, and besides, it can be kind of difficult to respectfully handle real life mental illnesses in terms of writing them. That's why I tend to avoid giving an actual name to any condition I give a character so I'm not accidentally being pretty terrible to some people by messing it up.

I mean, this might be a stretch, but maybe one side is like a ghost or magical being in her body or something if you want to play a character with two personalities?