Finn is Puck's younger brother, and he's so loyal to his sister that we're surprised she doesn't lead him around with some reins like she does her horse. He makes a huge sacrifice for Puck: Selling his car so that they can buy food for Puck's horse. This is a huge risk because there's no guarantee on his investment. It's difficult to tell if he's loyal or foolish when he puts all his eggs in one basket and bets his remaining money on his sister to win the race.
When he isn't putting his sister first, he's either obsessively washing his hands or compulsively cleaning the house to put his anxiety out of mind. We're not sure how a family ended up with a single-minded, hard-headed sister and a nervous wreck of a brother but, hey, someone has to do the dishes.