Overweight, asthmatic, intelligent, annoying.

He's the kid you like not liking in school.  There are a million little things about him that make him "different" from other kids, but really bugs people is when he thinks up a good idea or plan instead of the actual "leaders".  Because of his appearance, he is a natural target for a classic bully like Jack.  In addition, because just about everyone can easily pick on Piggy, he becomes the butt of any joke that can make everyone laugh and bring them closer together when things are on edge.