Martin Mertens

 Martin Mertens (or Martin the Human) is Finn's biological father. Billy calls him "Dad the Human," similar to Finn's title "Finn the Human".

History
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 is trapped in a prison leading Finn to believe that he committed a galactic crime. He runs away from the problems at the Citadel as the Lich spreads his evil leaving his sun. 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 part of Finn's arm.

In "The Visitor," he is seen to be living with a bunch of villagers and manipulating them to believe in a tree spirit in order to help him build his escape pod. Finn is angered with him by the end of the episode for treating the villagers so badly. Finn then pulls a switch on Martin's now fixed escape pod that sends Martin flying into the air, without the roof of the pod closed fully to keep him inside.

Appearance
Martin is an overweight man with a long, dark blond beard and a mostly bald head, with the exception of a single, curling hair at the top, making him resemble his Farmworld version. 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. In "The Visitor," his leotard has been somehow torn, leaving just enough to cover his lower body and his chest is left bare. He also wears a green military-like jacket and his hair in a loose ponytail.

Personality
Unlike Finn's selfless and moral personality, Martin is revealed to be an incredibly selfish, exploitative and narcissistic individual who barely acknowledges his son's presence unless it benefits him. From the moment he is freed, he is focused solely on saving himself, more shocked at the lack of escape than meeting his biological son, Finn. The extremity of his selfishness is shown when he does not even have a reason for abandoning Finn (suggesting he simply left his own baby son to die in the woods without any remorse) he simply says he doesn't remember, even attempting to shift the blame to Finn by saying "maybe YOU left ME," revealing just how immensely immature Martin is.

Martin is also a very emotionally barren person. He has absolutely no empathy and does not seem to understand how his actions affect others, such as treating Finn's emotional attempts to bond with little more than irritation and ingratitude. He then proceeds to manipulate Finn into fixing his leg by baiting him with paternal affection. Once his leg is fixed, he once again abandons Finn, completely unconcerned that Finn loses his arm in a desperate attempt to stop him. He also showed a total lack of sympathy for one of the collapsing Citadel Guardians. Finn at one point reflects that maybe Martin "isn't a dad, but a kid in a dad's body." Further evidence of this is shown in Martin's later appearances, his childishness suggested in his obliviousness to the total cruelty of some of his actions, such as trying to hug Finn right after lying to him about losing his arm, or getting Finn into a leglock right after telling him he never came back for him after leaving him in the forest.

However, like Finn, he is shown to be very persuasive, as he was able to convince the other escaped convicts to make him their leader. Martin does this using the same falsely charming smile and wink he used to get Finn to help fix his leg, emphasizing that, to Martin, others are just interchangeable assets to be exploited or manipulated for his own survival or gain and his son is no exception.

In "The Visitor", the Martin's manipulative and utterly insensitive nature is further explored. When calling Finn up into the tree that he has been living in since the crash, he seemingly has lost an arm, but after ensuring that Finn has 'gotten over' the arm losing incident, he reveals that he still has his arm, meaning that he was not only aware Finn lost his arm, but was deliberately mirroring it to gain Finn's sympathy so that his son would not try to to attack him out of revenge. This innate cruel streak in Martin is also signified when he is eating a birds egg, and spits out feathers, implying he was casually eating a hatchling.

Also in "The Visitor," Martin confirms that he is, in fact, Finn's father, and reveals that at that point, he had a personality similar to Finn's, as he defended Finn from the sea creatures trying to eat him. At some point, he was given a choice, and he couldn't bring Finn along; though he always intended to go back for Finn, he never did. However, it is unclear if this is actually true, as the previous flashback of Martin colliding with a tree was revealed to be a lie, and also Martin heroically defending baby Finn seems out of character for the selfish, disappointing man who was utterly indifferent to ripping his own son's arm off during his prison escape.

Issue about parenthood
Meeting Finn Mertens' birth father is a situation the boy can’t deal with by swinging his sword at a "bad guy," so this issue keeps him unable to move forward. When Finn finds out that The Citadel is a prison, he naively assumes that Martin is the warden, and Finn’s reluctance to draw negative conclusions about his absent parent sets him up for major disappointment when both meet.

Martin appears to be a darkly comic subversion of most absent parent storylines seen in cartoons or even anime; when the child is finally reunited with the parent, they often find out the parent left or abandoned them unwillingly in order to protect them from something or in the service of some greater cause. Martin, however, does not have a reason for abandoning Finn, and probably did it simply because he could not be bothered to raise him. This created both sadness and hatred in Finn, an issue that was shown in the episode "The Tower."

In "On the Lam", it is suggested that he was on the run from something when he originally had Finn as a baby and that he was forced to abandon him in order to save his own life. This shows that Martin does have a sense of heroism and protecting his son, but is overshadowed by his cowardly, crooked nature which makes it seem like abandonment for the sake of self-preservation.

Major appearances

 * "Finn the Human" (Farmworld Martin)
 * "Jake the Dog" (Farmworld Martin)
 * "Escape from the Citadel"
 * "The Visitor"
 * "On the Lam"
 * "The Comet"

Minor appearances

 * "Astral Plane"
 * "Hoots"

Mentioned or pictured

 * "Billy's Bucket List" (debut in Ooo)
 * "Wake Up"
 * "The Tower" (hallucination)
 * "Breezy"
 * "The Pajama War"
 * "Jermaine"
 * "Be Sweet"

Abilities

 * He is very resistant to pain, like his son, Finn. When he is shot by a laser from a Citadel Guardian in his leg (causing it to melt his flesh, only shows bone), 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

 * Martin originally was supposed to appear in "The Lich" after Billy told Finn about him; however, the postmortem spirit of Billy eventually tells Finn about Martin's existence in "Billy's Bucket List."
 * As seen in the original storyboard of the episode "The Lich" in the scene 118/panel C, it's possible to read in the description of the holographic animation about Billy's hands pulling Martin to the Crystal Citadel, presuming that Billy knows about Finn's dad because he was the one who put him in the Crystal Citadel.
 * Jake is shown to dislike Martin, referring to him as a "loser."
 * Martin's beard looks similar to the elderly Finn's beard in "Puhoy."
 * Martin's final words before disappearing into the void were "Hey, what about air?" implying that he cannot survive in space like the ancients and may have possibly died upon entering the void. This was confirmed to be false at the end of "Astral Plane."
 * Since his first appearance, Finn is shown to hate Martin, even attempting to take his arm as revenge for losing his own in "The Tower," though Finn still refers to Martin as his "dad," but in an unfriendly way.
 * However, Finn no longer despises Martin, but doesn't necessarily like him either. He now calls him 'Martin' more often than 'dad.'
 * In "The Comet," he states that he can't keep track of time consistently as he "doesn't have a star to revolve around," implying that he spends more time outside of Earth than he does on it.

Cultural references

 * Martin's red leotard is likely a reference to Sean Connery's various ridiculous and revealing red outfits in the 1974 science fiction film Zardoz.