I've never had a roomie myself, since I still live in a basement with my parents (okay I don't live in a basement), but through my journeys visiting friends I've reached a conclusion:
There is always, someone in the house, who is a mistery to everyone else, or is avoided by everyone else, throught my journeys I've met up (or not met up):
The Roomie who is never seen but only heard: This roomie you know lives there, pays her fair share of the house, and sometimes even people goes to visit her, but you have never seen her, she ussually screams too, you can hear her voice from upstairs asking you for something, or telling you and your friends to shut up, but she never appears.
The Roomie who dissapears for days: Now this one, you may ocassionally see her and exchange a few words, but you don't know anything about her, her room (or part of the room) is ussually empty or says little about her. You might go to sleep and wake up and she hasn't arrived, or arrived and went away without you noticing. You know she lives there, but it is as if she didn't.
The tall roomie who takes hours puting on makeup and is afraid of sleeping alone, also, never cleans: Well I know of all of this in one roomie. She is really tall, but is scared of sleeping alone, so she goes to someone else's room and invades her private space. She also takes hours putting on make up, so she is always late to school because she can't go out without make up, and then asks you to give her a ride, making you late as well. She also never cleans, and gets angry if you ask her to, so it's kind of the person you want out.
The freak brothers: This guy is a total geek, but not your ussual friendly geek like me, no, this guy scares you, he talks weirdly, makes bad jokes, and is ussually like a grown up kid. Everyone else in the house avoids him and sometimes even makes plans to get past him. He also has a younger brother, who is as geeky as him, but thinks he is superior, so he's always moking him and challenging you to videogames he knows you'll lose.
The heartbreak roomie who sings on her webcam: This roomie broke up a while ago, but she is still in love with the dude, so she invites him over and makes him dinner, even tho the guy already has another girl. The guy lives there as if it were his house, so everybody else in the house can't stand it. This roomie can also be heard singing in her room passionately, apparently via webcam with someone else, you just laugh silently and avoid.
The Milkless cereal eating Roomie: This guy eats cereal, with no milk. He may drink milk later, but never with the cereal.
The weird-noises making roomie: This roomie brings his/her couple over night, and you can hear everything on the next room (or in the same room), but you just remain silent, waiting for everything to stop.
The Closet roomie who is secretly in love with you: This guy is gay, but he hasn't told anybody. He invites you to sleep with him, gives you massages, tells you you look good, and is extremely friendly. Since he is your friend and you don't know anything about his secret desires, you accept everything, and live happily ever after. Note: I respect all gay people and support gay rights, this is just to complete the list.
All this are based on real facts. If you know any other kind of weird roomie, let me know, and I'll add it to the list.