Yeah, yeah, we know, Charles and Marcee are also characters. Well, they're characters who are actually delusions, but in any case they're definitely not your typical "symbols."
ALICIA: Charles isn't imaginary. He and John have been best friends since Princeton.
DR. ROSEN: Have you ever met Charles? Has he ever come to dinner?
DR. ROSEN: I phoned Princeton. According to their housing records, John lived alone.
However, they represent a huge part of John's early adult life that he has to reject in order to try to get better.
You see, John doesn't like the therapies used to treat his schizophrenia, so he goes off his meds…which means that he has a lot of trouble telling the difference between what's fake and what's real.
He's committed to figuring out a way to function without the drugs, though, so he finally resolves to just tell his delusions to go away. And those delusions include his old roommate, John, and his adorable niece, Marcee.
It's super hard, of course—and sad. He has to reject the "guy" he thought was his best friend all these years, and who was a kind of support system for him. Remember, Charles was his friend when no one else was. And he has to refuse to talk to Marcee or give her hugs when she throws her arms open to him. It's truly heartbreaking to watch, even knowing that these people are fake.
But those moments definitely serve an important symbolic purpose: they really drive home just how strong John has to be to get better.