Nice Guy: Lets see, he apparently buys Lisa a dozen red roses every time they make love, or at least often enough to be the flower shop lady's favorite customer, and would do anything for. He's paying for Denny's college and tuition without asking for anything in return. Is incredibly nice to all of his friends and seems to only want to make Lisa happy.
Took a Level in Jerkass: Towards the end of the movie, starting with the fight between him and Mark as he discovers that Lisa is cheating on him during his birthday party.
Ambiguous Disorder: His behavior is... incredibly bizarre for someone who is likely pushing his twenties. Even Tommy Wiseau has admitted the character makes more sense if you think of him as mildly retarded.
Ambiguously Bi: Reacts in a conspicuously nonchalant manner upon hearing about Mike's "underwears" and later expresses how "handsome" every other male character looks in their tuxes individually, sounding decidedly excited while doing so. Also, early on in the movie he explains to Lisa that he "loves" her...and Johnny after asking to kiss her.
Compressed Vice: His drug dealings with Chris-R are brought up once and then never again afterward.
Man Child: To a degree. He keeps trying to go upstairs with Johnny and Lisa when he knows they're going to have sex, and presumably wants to join in, then follows them upstairs and jumps on the bed. He also talks about how he's "in love" with Lisa like a 7-year old, and fawns over her to a creepy effect.
Kyle Vogt had another gig at the time of production, and despite repeatedly telling Wiseau that he had a limited amount of time to work, they were not able to finish filming his scenes. In fact, Wiseau insisted on the famous "football in tuxes" scene, rather than filming Peter's scenes at the party.
Remember the New Guy: A really lazy example. With about 20 minutes left in the movie, a new character, he suddenly appears and becomes deeply involved in the plot. Presumably, we're not supposed to notice that he suddenly appeared from nowhere without an introduction. Word of God is that Steven is supposed to be a replacement for the psychologist character Peter (whose actor left the production), but the audience is given no hint of this; indeed, Steven looks nothing like Peter.
Claudette's friend, who wants to buy a new house. Claudette asks Johnny to help her with a down payment, only for him to tell her it's an awkward situation. She expected him to be more generous. The only character in the entire movie to have a surname, yet she's entirely offscreen.