At the begining of the series, Sokka was credited with the title of comic relief.
Next, when Iroh came in, he was supporting the comic relief on the??fire nation??side, so you could have fun on both, (Book 1, Chapter 13 Iroh:"I win! But you are only improving, I am sure you could win if we... play again.")
Aang, because of his nature, was a side comic relief.
In the second season, it is never really thought as it, but Ty Lee and Mai are sort of a comic team, with Mai as dry humor and Ty Lee as a happy-go-lucky nonsence girl.
Later in the series, Iroh became more serious and his position as a fire nation comic relief fell. And he was replaced by Toph.
Because of her blindness, she makes fun of it and others, particually Sokka, fall for it (Book 2, Chapter 17 Aang: "That doesn't look anything like Appa" Toph" It looks just like him to me." Sokka:"Why Thank y... why do you feel the need to do that?"
And placed in randomly are smaller comic reliefs: Foo Foo Cuddly Poops, the Cabbage man, Mouth foaming guy, Bosco (the bear), Stupid Dai Li agent, and much, much more.
Name the others I forgot if you want to.
