First off, when you're 12, relationships aren't really that serious. They may SEEM that way but when you look back later on, you'll realize it's NOT that serious. Secondly, he's your best friend. For all you know, SHE kissed him. Until you know everything, don't make any moves to break up with her or stop being friends with him. Third, even if it WAS him that kissed her, don't stop being friends with him because of it. Tell them you're happy for them and know on the inside that it's not serious anyway because YOU'RE ONLY 12. When you're 16+, we'll talk.