1. Calm the hell down.
It's okay to be protective; but your girlfriend/would-be girlfriend is probably some crazy badass fairy/vampire/werewolf/unique sparkle pony or something. I'm PRETTY sure she can take care of herself most of the time. When she needs your help, step up to the plate. But don't try to do everything for her.
If it's an option, come in through the front door. Seriously, it's not that difficult to just knock like a normal person.
3. Make badassery contagious.
If your girl doesn't know how to fight and you're in some dangerous world, teach her. Don't have her depend on you because chances are, you're not always going to be around!
4. Be honest about your evil.
If you're an enemy in disguise, please don't keep it a secret to announce AFTER you begin your torrid love affair with the heroine and then EXPECT HER NOT TO BE ANGRY. That's nice that you had a change of heart and came to the good side but seriously, never a good idea.
5. A mask doesn't make a whole new person, stupid.
Start recognizing people. You see a person every day of your life and suddenly they put on a mask and you're like: oh ma god, who could this be? I don't know!
You might think being a douchebag is cute, but it's not. Other things that aren't cute: bad attitudes, controlling personalities, trying to shank your future girlfriend, and stalking.
7. Being taken doesn't usually mean you're waiting for someone better to come along.
If you already have a girlfriend, JUST WALK AWAY. Or dump her right now. Those are the only two options I don't care what you say.
8. Stop being boring.
BE YOUR OWN PERSON. When I can tell you're just a cheap plot device to pretend that the heroine actually has some decent character development, I want to kick myself in the shin. I want to know if you have hobbies, family! (An opinion of your own would be nice too but I don't want to ask too much of you)
9. There's already one Edward Cullen; we don't need more.
I get it, you're tortured; it sucks. But if you're going to spend the whole novel acting like a pissy little boy, I'm not going to like you. And that other guy with the abs is totally going to steal her from under your nose (JUST SAYING.)
10. GOD JUST STOP IT I CANNOT DEAL WITH THIS.
Stop dying. Or pretending to be dead. JUST STOP.