The Coal Knight, who's real name is Edan, is a character who first is introduced in the episode Runaways. He is a knight who wanders around in the forest of Ooo. He once was a knight in the Fire Kingdom but left when Flame King took it over and imposed a quasi-dictatorship that saw the kingdom become a belligerent military police state.
Nothing much is known how he really looks like since he has hid himself in a coal armor. The armor has a coal plum on the triangle helmet, and is very heavy, but because Edan had it on for such a long time he got used to it and can move faster. There is a orange flame spotted on his armor and some lava is also dripping out.
Edan being a fire elemental is able to bend fire but he said he hasn't done that since the fall of the original kingdom. He does have shown that he is very good in swordfighting. He has a giant axe, that is made out of coal, and can deal powerful hits with that.