Martin Mertens is the main, later secondary antagonist of season 6 of Adventure Time. He is Finn's biological father who doesn't really care about him at all. He abandoned his son in a forest and was subsequently imprisoned in the Crystal Citadel for committing a "cosmic crime". He is voiced by Stephen Root.
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 needed 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.
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