Flame King is a character that appeared in "Incendium." He is the ruler of the Fire Kingdom and Flame Princess' father. He seems to be very aristocratic. The king's favorite thing used to be a koala bear, but his fondness of them disappeared after Jake tries to transform into one. He referred to Flame Princess as his evil daughter. It is unknown if he has any relation to the Fire Count, who was defeated by the legendary hero Billy. After he demands that Finn and Jake be executed for apparently disguising "treason" as a play, Torcho and Furnace (his nephews) are discovered as the true conspirators, and they reveal that Flame King killed his own brother (Furnace's and Torcho's father) so that he could take the throne. The Flame king was overthrown by his daughter and Cinnamon Bun and placed in the cell lamp that he held his daughter in "Earth & Water". In "The Red Throne," the Flame Lord frees him in return for Flame Princess' hand in marriage. After Flame Princess refuses, however, Flame Lord and Flame King have a fistfight and end up imprisoned in the lantern together. He then insults the people for choosing his daughter over him, calling her a "softie" who has lost much of her power. Cinnamon Bun disagrees, saying that while Flame Princess is not ruthless, she is strong-hearted, and he has learned much from being around her. He promises to stand by her side as her knight and champion, because he loves her. She thanks him and they hold hands. When the fire people rejoice, the Flame King is outraged.
He has a slightly impatient and demanding personality, stereotypical of royalty. He is also very protective of his daughter, Flame Princess. However, since she's left the Fire Kingdom he hasn't appeared to try and bring her home although this could be because she is hanging out with Finn. He also understands himself as pure evil, having ascending to the throne by killing his own brother and understanding that this evil nature exists in all in the Fire Kingdom.
He is much bigger than the other fire people. He wears a copper armor suit, carved with rhombus-shaped red gems in the armor's collar with a bigger pentagon-shaped one in the chest. The limb pieces have horizontal stripes with copper and brown colors and his hands are uncovered showing his hand-shaped flames. His head is similar to a giant fireball inside the armor, there are three holes in the flames that form the eyes and the mouth. His crown has five tips and a red gem in the middle. When traveling greater distances, he gets out of his armor as a flame and the armor follows him to the destination. Unlike his daughter, he does not have a humanoid body.
When traveling greater distances, he shifts out of his armor as a flame. When he reaches his destination, he merges with his armor at the new destination. Like his daughter, he may also use fire-like attacks. He is acrobatic as he was able to jump from his balcony seat in the theater, do a full flip in the air and land perfectly on the stage in "Ignition Point."
- Flame King shares the same voice actor with Chaos from Final Fantasy, Keith David. Coincidentally, both characters are related with fire and destruction.
- In Burning Low it is implied that he may know Princess Bubblegum, as she was the one who had him put Flame Princess in her lantern.
- The reason he has his daughter kept in the lantern was because of her physical unstability. Extreme romance would have resulted in disaster for Flame Princess and the world.
- He claims to hate his jester.
- In "Ignition Point" he is revealed to be evil, and also confirms Flame Princess to be evil as well, but admitted that she can be become good from her experiences.
- It is possible that he's using his daughter's candles to leave mental suggestions that she's evil while she sleeps at night.
- He continued to harass Flame Princess by calling her evil, as seen in the beginning of "Vault of Bones."