Complex sentence: Nelson was introduced in Season 1's "Bart the General" as a bully but later turned into a friend of Bart Simpson, who is best identified by his signature laugh ("HA-Haw!").
Split into simpler sentences: Nelson was introduced in Season 1's "Bart the General" as a bully but later turned into a friend of Bart Simpson. He is best identified by his signature laugh ("HA-Haw!").