We're not sure why Kate goes by Puck—she's hardly a Shakespearean trickster. Maybe it has to do with her bad attitude that we always hear about, but never actually see. She describes herself as "cross" a good half-dozen times, and even Sean says she's a "cross child" (26.15) at one point. But other than being angry with her brother Gabe for leaving, we only see Puck in one mood: Determined to win the races.
She has a single-minded focus to win the race money and save her family home. All of her actions are steps toward this goal. Even her relationship with Sean begins because he offers to help her in the race.
We're impressed by how determined she is, because her road to the races is not easy. Everyone hates her because she's a girl, plus she can't ride a water horse. Her horse, Dove, looks mighty tasty to her water horse rivals, and she has to do all this with the threat of eviction hanging over her head. She deserves an award just for making it to race day without quitting.
In addition to this insane level of commitment, Sean and Puck have something else in common: Their best friends are horses. Puck cares for Dove just as much as Sean cares for Corr. When they start to fall in love after eating dinner together, you can see they each have a mutual respect for one another.
We don't mean this to be insulting, but another reason Sean falls for Puck is that she's kind of like a horse. You've heard the old carrot-on-a-stick analogy. For Puck, winning the races is her carrot, and she'll do anything to get it. Plus, she has the athletic prowess and intelligence of Sean's four-legged friends. Sean is shocked to see such horse-like qualities in a person, saying, "I feel disquieted [...] because of the girl, interrupting the routine I've followed for years" (17.10).
It helps that Puck isn't a girly-girl, either. Princesses don't go by the nickname Puck, generally speaking, and Puck is hardy peasant stock. While her life may not be glamorous, she doesn't wish it were. She and Sean have simple dreams: They want to live on the island with their horses. And by reaching for these (low-hanging) stars, they get exactly what they want out of life, together.