Kellen is prickly and awkward. He grew up poor and unpopular, and his family, while loving, is far from perfect. He has an inferiority complex that flares to life at the slightest provocation. Jamie is a charming, good-looking extrovert who seems to have everything going for him. Yet it turns out to be Kellen who has the stronger sense of self. He can't tolerate dishonesty or lack of integrity, and this is what drives the course of the romance.
Jamie is as much a product of his background as Kellen is, though it isn't as obvious at first. He has grown up in a community with a secret - they are "awakened", meaning they have powers other humans don't, and they keep this hidden. Thus Jamie grows up learning to hide who he is and lie by omission. In Jamie's case this habit of concealment is compounded by the fact that he is also hiding the fact that he is gay from his community. He never reveals himself fully to anyone. He is buddies with everyone, but at the start of the story no one is close enough to him to know all of who he is. So Jamie is living a shallow and lonely existence under his facade of friendly good cheer. Kellen, for all his grumpiness and awkwardness with people is the one who is known for who he is and relates to people honestly. But he is also so defensive and self-protective that he pulls away and endangers the relationship. He needs to learn to trust and forgive.
This sets up the best kind of romance - where the main characters must grow and change in order to become each other's match. No one is really in the wrong, and their weaknesses are understandable, but they have to decide whether to stay as they are or to do the hard work of changing - and accept the pain that comes with it.