1. Don't give him any unique qualities.
Ever. Especially not a cute crooked tooth or a loud laugh. He has to be tall and he has to be drop-dead (maybe literally) handsome. He needs to be brooding, he needs to be good at pretty much everything. Like playing the flute and counting to a hundred in Italian and standing on one foot for hours on end. Stuff like that. Hey, I didn't make these rules. This is what a perfect guy looks like.
2. Make him mysterious.
Later reveal him to be some misunderstood creature, like a werewolf or a... mermaid. Hey, those are bad-ass creatures. They're all bubbly and... scaly. Anyway, he has to be ashamed of what he is. So much that he's doing everything he can to be human. He will try to eat your main character at least once. Just a nibble, though. It's unavoidable. He may be perfect but he is a wild, untamed creature of hotness!
3. Make him fall for your main character instantly, for no obvious reasons.
Oh, and make him so intense that he won't even leave her side to let her pee. Yeah. That happened. And at some point he will make a long, romantic speech that requires no pauses for breath and will render your pretty main character unconscious with disbelief and joy. He's so perfect that he will of course have smelling salts on him. Oops. His fin just smacked her in the face. Oh, well. He's perfect, and this is empty love, what can you do?