Greg
A real asshole! He flirts with your girlfriend, and next thing you know, she leaves you and runs off with him! Beware of Greg's! They have a natural ability to hypnotize people (especially women) into doing what they want! They use tactics that are not approved of in the most advanced civilizations! Such as preternatural tricks of drugging women, and tricking them into becoming the Greg's mental slave! Or, being super nice, and super fake, which convinces most women, he's "a good guy"! Greg's are usually ugly, brutish, disfigured phantom men, who pray on younger girls and women who are 5-10 years their junior! Greg's simply want to shoot their demonic seed into as many women as possible to over run the Earth with demonoid bastard children, so as to solidify their empire! Greg's are all evil! Avoid like the plague!
(Girlfriend): Who's that guy over there? Do you know him?
(Boyfriend): No idea, never met him before in my life!
(Greg): Hey I'm greg, you want some drugs to get high?
(Boyfriend): No thanks...
(Girlfriend): Yeah sure!

20 mins later, Girlfriend and Greg are swooning over each other in the most vile ways, while Boyfriend is left alone sulking over his stolen love!