Ice King
Originally in the first seasons, he wanted to kidnap Princess Bubblegum on a regular basis. As the show progressed, he became a kind-hearted but misunderstood old man with completely twisted senses of morality and social behavior. He suffered from memory loss. Later seasons unveil his tragic backstory, revealing he was once a good human named Simon Petrikov who lost his mind, memories and loved ones due to the magic crown giving him his ice powers which was seen in the episode "Holly Jolly Secrets".