Martin Mertens

Martin is Finn's biological father. Billy calls him "Dad the Human," similar to Finn's title "Finn the Human." He is the only other known human living in Ooo.

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 difficult to enter and 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 silhouette of a man trapped in a crystal aboard a massive space station.

In "Escape from the Citadel," he is finally revealed, but has no clear memory of his son, Finn, or why he deserted him. He runs away from the problems at the Citadel as the Lich spreads his evil. He 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 Finn's arm.

Appearance
Like Finn, Martin 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 wears a red leotard-type outfit with red spiked shoulders. He has red boots that almost go up to his knees.

Personality
He is selfish and cares mostly for himself, as seen when he abandoned Finn on the collapsing Crystal Citadel and when he showed no remorse for one of the collapsing Citadel Guardians. Like Finn, he is shown to be very persuasive, as he was able to convince the other escaped convicts to make him their leader.

Abilities

 * He is very resistant to pain, like his son, Finn. When he is shot by a laser from a Citadel Guardian, he says, "It's okay. It's okay."
 * He is also talented at climbing and swimming, as he was able to climb a long vein and swim through the molten center of the Citadel.
 * Like his son, Finn, he is incredibly persuasive, convincing the ancients (Demons that have committed cosmic crimes) to make him their leader.

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 not have any real interest in Finn or to explain why he abandoned him in the first place, as he simply used his status as his father to make Finn help him and avoided answering Finn's question.
 * 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 as stated by Finn.
 * This could also explain the selfish and uncaring personality he displayed toward his son, Finn.
 * There is a possibility that Martin may not have known about his son, even with the revelation that he had a son. Martin showed little care for him, unless it helped him escape.

Gallery
Martin