Lumpy Space Princess, or LSP for short, is a character in Adventure Time and while more of a spoiled brat (and a stereotypical teenage girl) and not a villainess in the show, she was the final boss in the video game, Adventure Time: Hey Ice King! Why'd You Steal Our Garbage? After Finn and Jake defeat the Ice King, LSP picks up a piece of garbage and realizes that it's an ashtray she made Finn for his birthday and he threw it away. She develops a major tantrum and decides to destroy Ooo by detonating the Lumpy Space Reactor Core. 

She eats some beans, grows huge and opens a portal.  Finn and Jake go in with Lady Rainicorn to try and stop her. LSP's main attack is shooting black stars at Lady Rainicorn in a pattern. Eventually, after dealing enough damage to LSP, the Cosmic Owl appears and uses his energy to turn LSP back to normal.

LSP calms down and then realizes she lives in Ooo and doesn't want to destroy it after all. 

In the series, LSP is the first girl whom Finn has an intimate experience with beyond simply kissing. Just how far they go, however, is not revealed, although Finn is shown lying awake in his sleeping bag shortly after the ordeal, seeming to regret his decision.