He sounds like a real jerk, although it seems more like bullying than harassing... same difference.\
The way I see it, there are four options:
A) Just ignore him. Eventually he'll get bored when you stop reacting and pick on somebody else.
B) Fight fire with fire 1. If he has any character flaws that are worth picking on (and everybody does), do unto him as he does unto you. I get that it might be difficult because of being an introvert but it doesn't have to be spoken, think scrawled messages on the bathroom walls or something...
C) Fight fire with fire 2. Every time you hear him say something about you, IMMEDIATELY fire back with a calm yet snappy remark about HIM. Calm so he knows he isn't getting to you, snappy just to piss him off.
D) Go to your school principal or whoever is in charge and tell them everything. Take some creative liberties if you have to.