I've always assumed he was ISTj because he typed as such on a quick socionics test, but I want to explain some of his behavior to see if anyone can give an objective perspective on him.
Here we go:
He's a fairly quiet guy, and doesn't intentionally socialize much. He definitely has social skills, and can hold a decent conversation, but tends to stay in the background when around a more outgoing person like myself.
He's a bit vain; he has a fancy-shmansy haircut, refuses to wear anything that isn't expensive, and tends to think he's the best at whatever he's doing at the moment. He's decidedly paranoid: he insists that I lock the door when I so much as go to the bathroom (which is right across the hall), and thinks that someone is trying to get him kicked out of school.
He's a decent athlete, but refuses to rise above whatever level of skill he naturally has. There is one exception to this: he's on the Hong-Kong international/Olympic ping-pong team, and apparently worked his ass off from age 8 until he came to the US in order to get to the level he's at now.
When it comes to schoolwork, he tends to take the same approach as with athletics. We have the same calculus class, and he absolutely refuses to do the homework because he insists he can learn it on his own (then promptly comes to me after class, asking me to teach him what the teacher was talking about).
We have problems when it comes to our room. I naturally tend to leave things lying around, which he has a problem with. Even when my mess doesn't drift over to his side of the room or affect him, it apparently annoys him, and he's constantly reminding me to "be responsible" and keep my own things neat.
He can be quite diplomatic when he wants to. When he sees a problem in our relationship as roommates, his first instinct is to approach me calmly and reasonably as to what exactly the problem is, and what we should do about it.
So, any guesses? Feel free to ask for any other details about his behavior; I'm quite sure I haven't got it all down here.