"Luke's the best swordsman in the last three hundred years" (8.15). He is the counselor in Hermes cabin, and he helps Percy a lot in Percy's first days at Camp Half-Blood, showing him the ropes and explaining that everyone at the camp is family. Percy thinks Luke, who is seven years older, is perhaps the coolest guy he's ever met.
Gradually, over the course of Percy's time at camp, he realizes that Luke holds a grudge against his own dad, Hermes. Luke does a good job of covering this up, but Percy still observes a strain of sadness in Luke:
I started to understand Luke's bitterness and how he seemed to resent his father, Hermes. (8.10)
On the very last day of camp, Luke lures Percy into the woods and nearly kills Percy with a lethal scorpion. He confesses to Percy that he was the original thief of both the master bolt and of the helm of darkness. He tells Percy that he is helping Kronos overthrow the Olympians. When Percy exclaims that the Olympians are their family, their parents, Luke responds,
"That's supposed to make me love them? Their precious 'Western civilization' is a disease, Percy. It's killing the world. The only way to stop it is to burn it to the ground, start over with something more honest." (22.109)
Do you hear the venom in his voice? Do you hear his anger? Luke feels rejected by his father, Hermes, and he wants revenge. Hermes never really checks in on Luke, and once he embarrassed him by giving him a quest to retrieve a golden apple from the Garden of Hesperides – a quest that another more ancient hero had already successfully completed. Like Annabeth, Luke is a year-rounder at Camp Half-Blood, and so his life is pretty limited. He doesn't get to see the real world. His only chance at a quest was a repeat of another hero's quest, making him feel like Hermes was mocking him and intentionally embarrassing him. Luke is a talented swordsman and fighter, but his talents are "wasted" at Camp Half-Blood.
In his last few moments at Camp Half-Blood, before leaving to serve Kronos full-time, Luke tells Percy that "all the gods know how to do is replay their past" (22.122). He is inspired by the idea of change, and he craves revenge on Hermes.