John isn't the cuddliest of guys, but even he can't resist Charles' charm and good humor when he arrives in their shared dorm room and introduces himself as John's roommate:
CHARLES: The prodigal roommate arrives!
CHARLES: Oh, God, no. [Turns off John's music.] Ugh. Did you know that having a hangover is not having enough water in your body to run your Krebs cycles? Which is exactly what happens to you when you're dying of thirst. So, dying of thirst would probably feel pretty much like the hangover that finally bloody kills you.
Basically, this guy is the opposite of John: he's a fun-loving party animal who likes sharing fun hangover facts.
John tries to stay crusty and aloof, but it doesn't work for long. The next day, when Charles is feeling better, he pushes John to buddy up:
CHARLES: Well, it's official. I'm almost human again. Officer, I saw the driver who hit me: His name was Johnnie Walker. Well, I got in last night in time for English department cocktails. Cock was mine, the tail belonged to a particularly lovely young thing with a passion for D.H. Lawrence. You're not easily distracted, are you?
JOHN: I'm here to work.
CHARLES: Hmmm, are you? Right, I see, crikey! Is my roommate a dick? Hmm? Listen, if we can't break the ice, how about we drown it?
And sure enough, that's what they do: John and Charles get drunk and get to know each other, and John comes to realize (as we already have) that Charles is great.
One slight problem, though: Charles isn't real. Ever notice how no one other than John talks to/interacts with him, even if they are out at a bar together? Well, that's because John is the only one who can see him. John is schizophrenic, and Charles is one of his delusions.
We know, we know—we're just as surprised and disappointed as you are. We had kind of fallen in love with Charles (and a cute niece named Marcee that he sometimes has in tow), so it's a bummer that he's not flesh and blood. But really, we're most sad for John, who has truly believed that Charles a) existed and b) was his best buddy.
Alicia, who has been hearing about Charles from John for their entire relationship, is totally floored when Dr. Rosen reveals that Charles is a hallucination:
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?
ALICIA: He's always in town for so little time, lecturing.
DR. ROSEN: Was he at your wedding?
ALICIA: He had to teach.
DR. ROSEN: Have you ever seen a picture of him, talked to him on the telephone?
ALICIA: This is ridiculous.
DR. ROSEN: I phoned Princeton. According to their housing records, John lived alone.
When John commits himself to getting better without the help of medication, he has to basically say goodbye to Charles (and Marcee) and resolve not to speak to them anymore, even though he sees them around. It's pretty heartbreaking to watch, since he's basically giving up his closest friend…but in order for John to get better, Charles has to go.