If Mill does have an older sister personality, that means he has DID or Dissociative identity disorder--which is a multiple personality disorder--and NOT schizophrenia. If this is indeed the case, it would make sense for a lot of things--Mill's complete transformation into another person and identity altogether is not something you see in a Schizophrenic person. Plus, if I remember correctly, Mill doesn't recollect a lot of things from when his older sister personality takes over--this could mean he is genuinely not aware of most things he has done.
All in all, I wish he does end up getting some help, his mother really messed him up and I don't think there would be a good, clean way to end his arc if he doesn't get some therapy and get better.
If Mill does have an older sister personality, that means he has DID or Dissociative identity disorder--which is a multiple personality disorder--and NOT schizophrenia. If this is indeed the case, it would make sense for a lot of things--Mill's complete transformation into another person and identity altogether is not something you see in a Schizophrenic person. Plus, if I remember correctly, Mill doesn't recollect a lot of things from when his older sister personality takes over--this could mean he is genuinely not aware of most things he has done.
All in all, I wish he does end up getting some help, his mother really messed him up and I don't think there would be a good, clean way to end his arc if he doesn't get some therapy and get better.