|The following information is from the Adventure Time comic book series.|
It may not be part of the same canon or continuity as the animated series.
Bad Guy Magpie was the owner of the Casino Castle. After Agent Princess lost all her money gambling she believed the legitimate casino was rigged to cheat and began messing with Bad Guy Magpie, who kidnapped Peppermint Butler when he visited the Casino Castle, believing him to be Agent Princess in disguise. She attempted to kill him but Agent Princess saved him at the last second and blew up the Casino Castle. Bad Guy Magpie proclaimed the Casino Castle would be her last honest venture.
She later joined Peppermint Butler's venture into the Lair of the Vault King as a master of disguise, despite not actually being one.