Even though Finn thinks Puck is doing something dangerous and stupid by signing up for the races, he still goes with her for moral support. Her other brother, Gabe, does no such thing.
I trust Corr more than any of them.
I should not trust him at all. (9.9-9.10)
Sean and Corr's bond is unusual. People bond with horses all the time, and it's no big deal. But Corr isn't any horse… he's a supernatural creature and from a species that is hardly ever to be trusted. Why does Corr remain so loyal to Sean while the other water horses will turn on their owners in an instant?
"Five years I've kept you alive on that beach. That's what your father asks of me, and that's what I'll keep doing." (10.72)
Loyalty isn't just something that comes with blood relations. In Sean and Mutt's case, Mutt's father has bought Sean's loyalty, and Sean is a good enough guy to stay loyal even though Mutt doesn't deserve it.
"The capaill uisce killed my parents. I'm not going to dishonor them by riding one of the water horses." (16.61)
Here we see that Puck is loyal to the memory of her parents. Later she reconsiders when she thinks that riding a capall is the only way she can win. Do you think her parents would forgive her if she did?
I can't go without Corr. (20.20)
Just as Puck is loyal to her family, Sean is loyal to his. So what if Corr is a different species? He and Sean have practically grown up together, and Sean wouldn't pull a Gabe and leave him behind.
I know what I say dictates who he puts Mutt on this November. I don't want to answer truthfully, but there is no point lying, as he'll find out eventually. (22.55)
Malvern and Sean's relationship is complicated. You'd think Malvern would be loyal to his own son, but he still puts Sean on the fastest horse. Malvern is loyal to one thing—money—and he knows Sean is his best bet at having one of his riders win the races.
"I'll speak for her."
Every face turns to where Sean Kendrick stands a little apart from the crowd, his arms crossed. (32.101-32.102)
This is the moment where Sean puts himself in Puck's camp. By speaking for her at the ritual, he aligns with her and, as a man of his word, Sean will never back out of this deal.
"How do you know there isn't a faster horse than Corr out there?" (33.32)
Sean doesn't care if there's a horse that's faster, or even if there's a horse that could fly, because he's loyal to Corr until the very end.
Those are my horses down there in this beleaguered stable, out there in that fearful night. But at the same time, they are not mine, too. (39.4)
Even though Sean doesn't own Corr and the rest of the horses, he would do anything for them. A lack of money doesn't mean there's a lack of loyalty.
My sense of injustice is truly ringing. (42.30)
Loyalty is like karma in a sense: give it, and you often receive it in return. Here, we see Puck march down the beach to stand up for Sean… something she may not have done had he not done the same for her ten chapters ago.
"Where will you and Corr be?" I ask. [...] "Right next to you and Dove." (52.52)
We get the feeling that Sean and Corr won't just be riding next to Puck and Dove in this year's Scorpio Races… they might just be riding next to each other in life.