I would say he was definitely acting selfishly at first, since he was overcome with hatred and bitterness. He was only trying to use YH and kept tricking her, and also stabbed her quite a few times, like ouch. But he slowly warmed up to her afterwards, after all the time she spent trying to protect him and help him, he probably felt that she could br trusted. But even when he fell in love, he resisted against it, cuz he fears weddings and love and romance. He overcame that slowly, took him a while to really get over that dark shadow of his past. We also saw him as a dragon king before meeting that bish, he was pure of heart, kind to all, and a good king.
Overall, I think his character is originally one of only pureness and goodness, which turned bitter due to betrayal, but healed with the love of FL. He is now no longer as pure, but has gone through tribulations and learned of the ways of the world. The way we saw him being naughty and sly, and claiming he 'learned from FL', I feel that is the perfect sum up of his character. Still good, but also learned to be just a bit 'bad' sometimes