Toph and Zuko
Toph is from the Earth Kingdom. She is Aang's earthbending teacher. She is blind, so she uses earthbending to "see" by using the earth to "feel" other things. She also invented metalbending, where she can use metal to "see" like she does with earth. She can even use metal to fight, like with earth!
Zuko is Aang's firebending teacher. He was the Fire Nation prince, and was banned from it by his father. At first he was on a quest with his uncle to find the Avatar and bring him to his father to gain back his citizenship, but finally turned good and joined Aang's league.