|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.|
Mnemonoid - Is a character presented in Comic Issue 31". He is an Alien of unknown species, he has several powers, he can break the fourth wall, he has telekinesis and he is able to take memories. Mnemonoid eats stories instead of regular food, he is able to get the stories from taking them from people's memories, or by someone simply telling him a story. He is also able to make an oasis in a sphere on a snowy mountain.
His first appearance he shows up on a page in the comic, claiming that the page is blank and he is using his powers to make the reader be able to see him, and informs you of his name and that he will show up soon and take something from Finn. He next shows up and steals Finn’s memories, then once Finn is in the future with only the memories of right before Mnemonoid attacked, they go and find Mnemonoid again, he then steals Finn’s Memories again. Finn then wakes up way later in the future as an old man, him and Princess Bubblegum go and fight him one last time, this time going back in time to give the Mnemonoid Lumpy Space Princess, before he is able to steal Finn’s memories the first time, Lumpy Space Princess being able to give him unlimited stories, Mnemonoid has no use for Finn. He is last seen telling the reader that this was the story of why he doesn't steal memories anymore, and just goes and chats with Lumpy Space Princess when he is hungry.
He looks like a brain with a suit, however his head is revealed to look more like Cthulhu’s head. He does have a mouth and teeth. He is not specifically evil however he does do some bad guy things at the beginning, but in the end is good.