The usually have to meet this criteria:
1. I have to like the franchise they're from. (Only franchise I don't like in Brawl is Fire Emblem.)
2. They have to have a decent learning curve and not be obnoxiously easy to play and win with *coughmetaknightcough*.
3. Not be freakishly overused. (The only exception for me is King Dedede.)
4. I have to like the characters. (I'm pretty good with ROB but I absolutely loathe him.)
5. I have to like their playstyle. (I don't force myself to play with anyone simply because they are top/high tier. For example, I can't stand playing as Snake.)
6. I have to have played at least one game in the character's franchise. (Which counts out MGS and Fire Emblem for me.)
7. Something about them makes me laugh when I'm playing as them. (One of the big reasons why I like Wario so much in Brawl.)
With this being said, my mains are Mario, Ness, Wario, Fox, the Ice Climbers, and King Dedede,