Nelson Muntz is the bully on The Simpsons and is voiced by Nancy Cartwright. Nelson is probably best known for his class "Hah Hah" laugh he does at everyone.
Though Nelson is typically foun bullying Bart and other students, we've also seen his softer side as Lisa's boyfriend and even as Bart's friend.
Goon #2: Nelson, you're bleeding.
Nelson: Nah, happens all the time. Somebody else's blood splatters on me. (Sniffs) Hey, wait a minute. You're right. (to Bart) You made me bleed my own blood!
Nelson [about the school's ancient computer]: My stroked out grandfather has more memory tha that thing
Milhouse: How's he doing?
Nelson: Better, thanks