The Rat King is character introduced in "Little Brother." He is, of course, the king of the rats. Kent went on a quest to defeat him by breaking off his teeth to protect Finn and Jake's tree house, and he apparently succeeded.
The Rat King is a white rat with a crown. The crown has a single blue gem in it. His tail is pink as well as his eyes, and he possesses long, glowing white teeth that are seen releasing a poisonous green smoke. In the final battle between Kent and the Rat King, the Rat King wears a cloak and has blue mist coming out of his mouth. When he was seen earlier in the episode, he appeared to be a normal rat but with a crown. Later in the episode, he could talk, and his body under the cloak was made up of a horde of rats. His teeth are hard enough to require a special sword to break, and his constant gnawing at its roots has prevented the willow tree from blooming.
- In his second form, the Rat King resembles Nightmare, the final boss of the NES game Kirby's Adventure.
- A "rat king" (or "Rattenkönig" in German, originally meaning a person who lives off of others) is a term referring to a mass of living rats who have become connected due to their tails becoming knotted together. A "king rat" was believed in medieval times to ride around upon the tails. This is similar to how the Rat King appears to be an intelligent swarm of rats with a crowned head at the top.
- Although this is likely a coincidence, the Rat King resembles the character Little King John from the web series Ratboy Genius.