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Martin Mertens is a character from Adventure Time, and is Finn's biological father, but he doesn't care about him at all.
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.
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.
- 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."