Okay, most people here say whoop his ass. You're 14, lets stay real here.
First and foremost USE YOUR ANGER AND HATRED!
I am going to explain to you step by step what you should do-
1. Think of him, the creep lurking around your girlfriend, the girl YOU LOVE, him planning of raping her, him smashing her window, him cutting her, him attempting to take her BY FORCE, THE GIRL YOU LOVE!
Hyped up and angry? Good, go to step 2.
2. Go into a trance fueled by hatred( it's something hard to do, but if you summon up enough anger and hate, you'll reach a trance-like state)
3. Focus on him in grey flames, with a black aura. Keep focusing on the black aura until his aura is completely black. Then let the grey flames consume him. The black makes him easier to attack through the grey flames.
4. Now prey to Father Lucifer that he guides you and your girlfriend through this.
5. Clean your own aura of all the grey energy or bad stuff might happen to you.
6. Call the cops, inform your mom and dad, tell ANYONE who can possibly help. Since you just cursed him(strongly, seeing your hatred) he has a high risk of getting caught and getting BAD stuff to happen to him.
7. (optional) work on your gf's aura, make it a protective one, calming her and keeping creeps away.
8. Relax, find your center, let it all flow out and go comfort your girlfriend.
Good luck friend, this is your first step to becoming a true warrior, since we all start through hate.
Ave Satanas