Ice King

 once was a human with the name of Simon Petrikov, before the Mushroom War. Petrikov was an archeologist, who discovered the ancient book Enchiridion. Now he is a wizard and over 1000 years old and rules the Ice Kingdom. Unfortunately, his power source, his crown, also steals away his sanity. He is also a reappearing villain in the TV-show.

In

 * Jake and Finn visit Simon during the quest Speak to the Ice King, when they have realised, that great potions of the Ice Kingdom have mysteriously molten and flooded the land. He is very upset and complains about his ruined kingdom, missing his crown and penguins including Gunter. Jake and Finn are convinced there is more to it and want to make Simon talk and introduce "Interrogation Time", some kind of mini game, where Finn and Jake play "Good Cop/Bad Cop".