Mine's just "blow up everything on the field, then prevent people from putting anything else there or upgrading whatever they have there." Getting stronger the fewer cards are on the field is a must, and stealing other people's card effects to boost myself might also be appropriate. Kinda fits, considering how antisocial my Warrior is. "I don't need anyone's help!" and all that. :P
I think I might actually go for a setup where the character's personality is drastically different from her Warrior's, but her Warrior is in control almost all of the time, and she might not even be aware of what she's doing during those periods. Her Warrior might not necessarily be an outright antagonist, but definitely the sort of edgy rival character that's mandatory in this kind of story, if that's at all possible.