This Article Contains Spoilers - WARNING: This article contains major spoilers. If you do not wish to know vital information on plot / character elements in a story, you may not wish to read beyond this warning: We hold no responsibility for any negative effects these facts may have on your enjoyment of said media should you continue. That is all.
Martin Mertens is a recurring character in Adventure Time and serves as the secondary antagonist turned anti-villain of season 6. He is Finn's biological father, but he doesn't care about him at all. The reason behind his behavior is because of the brain damage he suffers in the past.
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 part of Finn's arm.
In "Astral Plane," it is revealed Martin found a spaceship after the events of "Escape from the Citadel" and was going to crash on Mars. Grob Gob Glob Grod mistook his ship for the Catalyst Comet and sacrificed himself by flying at the comet and sending it on a course for Earth instead of Mars.
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. The craft is named 'The Minnie'; possibly the name of Finns biological Mother. 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.
In "On the Lam" Martin has made his way to a new planet, once again getting himself imprisoned for betting on "lard-fights." He escaped with a Koala Person who took him to his home with many Koala People. Martin, disliking the food, offered to go to the outside world and get them some real food. This was most likely a lie however, since Martin sold their Space Moth and went to a bar. Martin named the Koala Person, Martin 2 and kept him as a pet. Martin and Martin 2 were recognized by the guards who attempted to capture "the rebel leader." Martin believed this to be him, and attempted to distract the guards away from Martin 2. When it was revealed that Martin 2 was the one they were looking for however, Martin abandoned him, regained the Space Moth and fled the planet.
In "The Comet" after Finn and Jake followed Orgalorg into space in Princess Bubblegum's spaceship, Finn was left floating in space. He was singing a song while Martin found him. Martin almost flaked and didn't save him but he did. Martin need to help Finn stop Orgalorg. Finn spoke to the Catalyst Comet that was going to hit Earth. The Comet gave Finn the option to start a new form of life. Finn declined the offer, but Martin took it up instead. Finn was initially upset with his father for trying to leave him once more but Martin told him, "No answer I give you will be satisfying." and, after being asked why he always runs from everything by Finn, "You burn enough bridges, the only direction to move is forward." Finn accepted that he could never change Martin, and Martin took off with the Comet and said goodbye to Finn. It is unknown if Martin is now a new life form or was brought to a new dimension.
Appearance and Personality
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.
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.
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", it is shown that Martin has a very cruel sense of humor. 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.
It is revealed in the flashback in ''Min and Marty'', he is actually a caring father. However, after saving Finn's life from the Guardian, he suffers brain damage and memory loss causing him to become a jerk like he is today.
- He's played by Stephen Root, who was the voice of Mildew in Dreamwork's Dragons: Riders/Defenders of Berk.
- His name, "Martin," closely resembles Farmworld Finn's last name, "Mertens."
- Martin appears to be a darkly comic subversion of most absent parent storylines seen in cartoons. When the child is finally reunited with the parent, he often finds out the parent left or abandoned him unwillingly in order to protect him 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.
- 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.
- Martin's beard looks similar to the elderly Finn's beard in "Puhoy."