Martin Mertens

Martin is the biological 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.

In "Escape From the Citadel," he is finally revealed and has no memory of his son, Finn, or why he deserted his son. He runs away from the problems being faced in the Citadel as the Lich is spreading his evil. He is the one who ultimately causes Finn's arm to be removed, as he tries to escape the Citadel while Finn is holding on to the escaping portal, causing Finn's grass sword to take over and remove itself and the arm.

Appearance
Like Finn, Finn's father has blonde hair, but is balding and his hair is a darker shade of blond. He also has a dirty blond beard and is overweight. In "Escape From the Citadel," he is shown wearing a red leotard-type outfit with red spiked shoulders. He has red boots.

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.

Trivia

 * He was originally supposed to appear in "The Lich" because Billy told Finn about him (eventually he had his first and official appearance in "Billy's Bucket List," coincidentally, Billy was the one who told Finn about him as well)
 * He seems to have no memories of Finn or why he abandoned him in the first place.
 * His name "Martin" closely resembles Farmworld Finn's last name "Mertens".
 * Although he was originally planned to be unlawfully imprisoned, it seems that he may actually be a criminal.