Lady Rainicorn is a female Rainicorn and is Princess Bubblegum's royal steed, best friend, and companion. She is also Jake The Dog's girlfriend, and sometimes follows Finn and Jake on their adventures. She speaks only Korean, using English only when equipped with a universal translator device (however, she is shown to have spoken a bit of English in "Lady & Peebles" "I am pregnant!"). Princess Bubblegum, Lady's children, and Jake are the only other beings in the Land of Ooo, besides Lady's parents, that are shown to understand Korean. Finn directly refers to the language she speaks as Korean in "The Pit." In "Lady & Peebles," Lady revealed that she is pregnant; this was confirmed by Princess Bubblegum. In the episode "Jake the Dad" she becomes a mother to five rainbow puppies.

In the version of the program that is broadcast in Korea, Lady Rainicorn's dialog – along with that of all the other characters – is in Korean, but the narrative nonetheless presumes her to be speaking a different language from the other characters, and her voice has both reverberant and polyphonous qualities added to it in order to differentiate it.

Somewhat interestingly, the exact phrasing of Lady Rainicorn's lines often varies somewhat between the American and Korean versions of the program. For instance, in the American version of the episode "Jake Suit," when Jake arrives at their home while controlling Finn's body, Lady Rainicorn asks Jake in Korean, "Why are you in Finn's body," while in the Korean version of the episode, she asks of Jake in the same line of the same scene, "Why are you in Finn's throat?"