Martin Mertens

Finn's dad is the birth father of Finn. Billy calls him "Dad the Human" similar to Finn's title "Finn the Human."

Reveal
Finn's father was intended to be revealed in "The Lich," as seen in unused storyboard scenes. He was a warrior who was unintentionally trapped in the Crystal Citadel, a prison that is easy to enter but nearly impossible to escape.

In "Billy's Bucket List," he was officially revealed, albeit briefly. Billy explains to Finn that his human father is trapped in the Crystal Citadel, now renamed "The Citadel." The episode cuts to a brief scene that shows a silhouetted man trapped in a crystal aboard a massive space station.

Appearance
Like his son, Finn's dad has very long hair. On the top of his head is a single, curled hair, making him resemble his Farmworld version. The color of his hair is unknown because he was only ever seen in the original storyboard for "The Lich" and silhouetted in a crystal. He is very large and muscular with thin legs, similar to how Finn imagined his adult self in "Mortal Folly." He 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 two different monsters with a sword in the Crystal Citadel.

Gallery
Padre de Finn