Martin Mertens

Finn's dad (Ooo) is a legendary warrior in the land of Ooo. He was unlawfully trapped in the Crystal Citadel and resides there, fighting off the inmates that reside there, as well.

Appearance
Like his son, Finn's dad has very long hair. The color is unknown because he was only ever seen in the original storyboard for "The Lich". He is very large and muscular and has only been seen wearing pants, socks and shoes, but no shirt or hat.

Abilities
Finn's dad seems skilled with a sword as well, as he is seen beating up 2 different monsters with a sword in the Crystal Citadel.