Furnace

Furnace is the nephew of Flame King and the cousin of Flame Princess.

His father was killed by his uncle in order to take the throne. As a result of this, Furnace and his brother Torcho attempted to assassinate their uncle by pouring ice over him as he watched a play, but were foiled by Finn and Jake who overheard them discussing their target and decided to intervene on their assassination.

Furnace speaks with a hiss inflection in his voice. Finn and Jake use this recognisable trait to track him down since they had yet to see his appearence after hearing him talk about his plot to Torcho, and this leads them to accidentally mistake Jalapeño pepper for him when a snake on Finn's shoulder hisses as Jalapeño pepper speaks. Eventually Furnace's hissing enables Finn to recognise him as the conspirator when he is disguised as an executioner.

After Torcho and Furnace's plot is exposed, they are both put in a "Punishment Room."

Trivia

 * Like everyone else in the Fire Kingdom, according to Flame King, Furnace is evil.