You said they were a main character, but how much of a main character are they if you haven't really got them defined in your head, and how could you get very far if you don't know how they'd react to things?
Sorry for being all picky and stuff, it's hard to repress my instinct for picking out stuff like that.
Anyway, here's this thing on that big site thing which might help [I've never actually tried it, since my characters are mostly defined in my head, I had made a bunch of character sheets once but they were disigned just to provide little pictures and be funny (with questions like blood type)]
http://elfwood.lysator.liu.se/farp/thewriting/crissychar/crissychar.htmlalso, I have a different thing which is for defining what the characters look like. You'd probably be able to try since you play Magic, I don't know if this works either since I don't have enough cards, it's my friends big idea. Basically, what he does is he looks through his cards until he finds one with an illustration that matches how he views one of his characters in his head and uses that rather then a shifting image in his head. I don't know if it works, but he's constantly going on about how great it is so I'd try it out.
