Looks Like Cesare: Black hair, and an appearance that suggests he doesn't go outside much.
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.
It's All About Me: Case in point, after her fiancee commits suicide, her first action is to tell the guy she was cheating on him with (who also happened to be his best friend) that they can finally be together. Mark rightfully calls her out on this.
Jerkass: Cheats on her loving fiancee with his best friend because she's bored with him, was implied to have just been using him for money in the first place, lies about him abusing her to justify it and tells his best friend that there is nothing between them after Johnny kills himself. At the end, Mark finally grows a spine and tells her off for it.
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.
The Mentor: The closest thing this movie has to one. Naturally, she's very bad at it.
Only Sane Woman: She may be an immoral, self-absorbed hypochondriac, but she's also the only character in this movie who seems even remotely aware of how absurd the plot is.
Pragmatic Villainy: She wants Lisa to marry and be faithful to Johnny so that she can live off a stable life with him. This ends up with an extreme case of Villain Has a Point: If Lisa had listened to her mother, it would have been for the better for everyone involved!Johnny wouldn't have committed suicide, he and Mark would still be friends, Danny would still receive college tuition and Lisa would, again, live a good life.
Straw Feminist: She has a very low opinion about men, but she's at least open-minded enough to know that women can also be manipulative.
Remember the New Guy: An impossibly lazy example. With about 20 minutes left in the movie, a new character 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.