Why does my friend do this?

Okay, so one of my best friends (we can call him D) is really wierd. He's a "popular" kid and everyone in 7th grade likes him. I'm a loser and I have a lot of other loser friends but he has cooler friends. He tries to get me to come over to the cool people groups, and when I do, he says "Hey Carse!" (my name is Carsen, which sucks because I'm a girl) and then sometimes he hugs me. In math class I'll bend over my desk and he'll come up behind me and grab my sides. It's really funny, but it scares the crap out of me and I usually scream or shout a cuss word, which the teacher doesn't like. I never buy lunch because my mom doesnt want to give me lunch money, and he usually offers half of his lunch. I say no thanks and then he says "If you don't eat this, I'll... I'll... I'll grab your sides in math!!" And I'm like "OMG FINE." He does have a girlfriend, and his last girlfriend and current both hate me. I don't know why but they just hate me. He'll come up and sit on my desk while I'm doing work or something and say "Hey Carse!" and his girlfriend will walk up while we're just talking and she'll hug him and give me an ugly face. I don't know why she would hate me, my other best friend Josh says she might be jealous. I don't know why she'd be jealous, because I'm fat and everyone makes sure I know, because they tell me how fat I am constantly. I'm not THAT fat, it's just that I can't wear clothes their size because it squishes my chest (I'm a D cup and everyone else in my grade is an A or B cup) so I wear guys T-shirts and they are really baggy and make me look fatter... But anyways why does D talk to me more than his girlfriend? Why does he treat me better than his girlfriend?! How do I get his girlfriends to stop hating me?!?!