Although his battle card team (the Blue Wolves) is known to be pushy and unfair, Gustav seems to be all about fairness....until he loses. He tries to keep his cards after he tells you he'll give them up, but because he forces Isabella to give up her Mew card, she guilts him into doing the same. He is a pushy and bossy leader.
His large buck teeth make him look like a rodent. When you come calling on Nelson's mansion after you reach A Rank, Gustav will bet his Ghost card in a match against you.