Too new a player to have keeping cards, but in the future, I suspect a lot of my keeps would be for favorite favorite characters in decks I have already mastered but selfishness refuses to have me share, or those special decks in which that character is featured once and I can't bring myself to collect the rest. ...And because once they're mastered, I display only the mastery badge and don't get to look at the pretty cards anymore, hah!

Taking fighter17~ :D