When a guy constantly comes over to you at school...?

I always stand at my friend's locker in the mornings and talk to two of my good friends there. Well there's this guy who always comes over, and I've noticed that ever since I started hanging out at my friend's locker every day, he's also started to come over a lot more. And he always stands either right beside me or directly across from me. I always catch him looking at me. He always listens to my conversations too. No matter which friend I'm talking to, he'll turn and listen to that conversation. Sometimes this guy will say something directly to me, but it's very seldom, and it's usually only a few words. Sometimes he'll move really close to me, so that we're basically touching. And today when he came over, he seemed to be sticking up for me when I was complaining about someone I don't get along with. I honestly don't get his behavior. He just keeps hanging around. What does this mean?