Fern is major antagonist in season 9 and season 10 of the Cartoon Network series Adventure Time, serving as the main antagonist of the Season 8 finale "Three Buckets" and the secondary antagonist of Season 10. He is an alternate timeline version of Finn that turned into the Finn sword and was eventually merged with the Grass Demon from the Grass sword.
He is voiced by Hayden Ezzy.
Fern first appeared at the end of "Preboot" when Finn's Finn sword combined with his Grass sword and created Fern.
He then appeared in "Two Swords" where he thought he was Finn and attacked him to defend Susan (who said ouch right when Finn touched her). When Jake told Fern he wasn't Finn her got confused and ran off. Fern then went to the Tree House where he started to remember how he came into existence. After Finn's fight with bandit princess both swords clashed into each other causing them to merge. Then came across a grass octopus spider (or the Grass Demon), which is actually the curse of the Grass Sword who cocooned the Finn in the Finn sword, transforming him into Fern.
Fern then started breaking things and scaring BMO. Jake and Finn then confronted him and the former started running around the house with Fern because he wouldn't look at Jake when he was mad. Finn then stopped the commotion by offering Fern a Finn cake. At the end, Fern is seen sleeping on the roof of the treehouse with his hand, imitating the grass sword curse.
He then appeared in "Do No Harm" where Jake tried to help him find his place. This led them to the lair of the Grassy Wizard who created the grass sword. Fern asks him for answers about his existence but the wizard just says he created the grass sword as a joke to the phrase "Blade of Grass." Fern then beats up the wizard and takes him to the Candy Kingdom hospital. Fern then steals starchy's bike then rides off into the sunset with his hair flowing in the wind.
Fern then appeared at the first episode of the "Islands" miniseries "The Invitation" where Finn meets him up on the tree house roof and entrusts the safety of Ooo to Fern. Fern then goes to the beach to see Finn set off and waves goodbye. Fern then appears in a flashback in "Light Cloud."
In the first episode of the "Elements" miniseries "Skyhooks" where he has been transformed into Fun by the candy elemental powers causing Fern to become so happy that he doesn't have any problems about himself. Fern is then transformed back into his true self in "Skyhooks II" by Lumpy Space Princess.
In "Whispers" Fern tries to help save Sweet P from the Lich's hand but fails. At the end he becomes more jealous of Finn and decides he should be Finn as he slowly transforms into Finn from the Grass Demon.
In "Three Buckets" Fern traps Finn in a Stone Temple and tries to take his place. Finn then gets out with his robot arm and fights off Fern. Fern then gets taken control of by the curse. Fern is then killed by Finn with his remains spread all around him. Fern remains are then picked up by Uncle Gumbald.
Fern then appears in Gumbald's lab in a new form in the episode "The Wild Hunt." Fern then makes his reappearance in "Seventeen" as the Green Knight. He competes with Finn in some birthday activities then he defeats Finn in an arm-wrestling match. He was about to kill Finn but was stopped by Gumbald. After a recap of things and an attack attempt by Gumbald, Fern leaves with Bubblegum's relatives promising to be back.
In "Gumbaldia" Fern fights off Finn but gets stopped by Aunt Lolly. In the end he is part of the group of villains that Uncle Gumbald has gathered to go to war with the Candy Kingdom and Princess Bubblegum.
Fern makes his final appearance in "Come Along With Me." He is ready to fight in the gum war but gets trapped in a dream with Finn and his friends. After Gumbald betrays him Fern starts to attack and destroy things. Finn tries to fight him to calm Fern down, but they both come face to face with the vault demons that Jake released. When Finn steps inside the vault he sees Fern as himself with the grass demon's curse strapped to him. Finn then manages to remove the curse, but before Fern kills it, the Grass Demon tells Finn that without him, Fern's got "no body" and Fern kills it, freeing him from it's control. Now fully redeemed Fern aids to fight off GOLB, but without the grass curse Fern is slowly deteriorating. Eventually, Fern dies and leaves behind a tiny seed shaped like the Finn sword. Finn then plants the seed and it grows into a big tall tree with the Finn Sword in it.
- He is fittingly named after a plant. A fern is a member of a species of vascular plants that reproduce through spores and don't produce flowers or seeds. (Although he did grow a flower in his nose when he smelled a Finn Cake)
- Strangely, Fern is unable to breathe (and can't play the flute) despite being able to talk.
- In the episode "Three Buckets," Fern wanted to be the only Finn and take over his life, which is a reference to a Star Trek episode, "The Enemy Within".