I need help with my mom's boyfriend's kid(s)?

So, my mom's boyfriend has 2 kids, Dennis and Cathy. Cathy is 14, and Dennis is 13. I'm the same age as Dennis. Cathy, I don't mind her, she doesn't bother me. But Dennis is nothing like me! He loves sports, going outside, ect... ect... I really enjoy video game and computers and such. I'm also not a big social person, so I don't make friends easily. I have a twin brother, so really, I'm with him. Dennis can sometimes be mean, and sometimes we have good times but i'm really sensative, and he sometimes makes fun of me and stuff. and I get really sad. I'm really emotional, and I have cried 2 times because I don't want to deal with him. It's also because of my parent's divorce, which is another story. How do I deal with him? I'm going to a football game with him tonight against my will, I did once before, and my brother and I just sat on a hill and talked throughout the whole thing. Also, this has been going on for about.... 5 months?