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Fern (full title: Fern Mertens the Human, formerly known as Grass Finn) is the result of Finn's Grass Sword and Finn Sword merging into one being. He makes his first appearance at the end of "Reboot."
In "Two Swords," it is revealed that Fern was created when Finn accidentally broke the Finn Sword with the Grass Sword while fighting Bandit Princess, during "I Am a Sword." Some time later a spider-octopus-like Grass Demon interacted with the small Finn inside the sword. The Grass Demon wrapped the sword Finn in goo, forming a cocoon around him. At an unknown period of time later, the interior of the sword was filled with green cobwebs and the cocoon appeared hardened. Both the sword Finn and the Grass Demon expressed desire to get out. The Grass Demon told the sword Finn to let it to do the talking when they escape.
Fern emerged in "Reboot" when the Grass Sword touched the Finn Sword again and began to cover it until Finn's arm detached from his body. When Fern first emerged he looked like a lump of grass with a face but then began to look a like Finn but made out of grass,even gaining the ability to speak. He then revealed that he believed that he was Finn, remembering everything Finn remembered. When he first saw the real Finn he thought he was a threat, believing that the real Finn was just someone trying to be like him. He than went to Jake and asked him to tell Finn who the "real Finn" is, to which Jake logically responded that Finn was the "real Finn." This upset him, and became more upset when Finn and Jake began yelling at him calling him a demon.
After this Fern went to Finn and Jake's tree fort (believing it to be his home). He than walked in and said hello to BMO. Confused by this BMO ran into a hole where Fern won't find him. Fern was confused (and kind of in denial). Fern started looking through a scrapbook of Finn and Jake's adventures admiring it. Later Fern was frustrated and confused and began to trash the Treehouse, scaring BMO even more. BMO immediately called Finn and Jake and the two came straight home. When they got back Jake was furious with Fern for scaring BMO and trashing their house, making Fern even more upset. Realizing the creature meant no harm, Finn offered him a Finn cake. Finn cakes are for "Finn only" so this made Fern felt more like himself. The group then went to bed and forgave one another for anything they may have done or said.
In "Do No Harm," Finn had to step out of Tree Fort to check on Susan, meaning Jake had to be alone with Fern much to his chagrin. Finn told Jake to pretend that Fern was him so and do "Finn stuff" with him. Jake made Fern meatloaf, which Fern tried to eat but learned his body was unable to consume food. This upset him since Fern still had memories of food and how it tasted. Fern was also unable to play the flute since he doesn't breathe. Jake then gave Fern the idea of going to an "evil dungeon to smash. Fern then took Jake to the Grassy Mountain where the Grassy Wizard lives. On their visit they fought grass monsters which made Fern happy, albeit a little more violent than Finn would. At the top of the mountain they saw the Grassy Wizard, who recognized Fern, realizing he came from the Grass Sword causing Fern to realize that he, made him. Fern then proceeded to ask why he (as the grass-sword) existed, only to learn he only made the Grass blade because it sounded cool and was a play on words, with no real dark plan behind him. When the Grassy Wizard asked Fern to kill Jake he than punched the Grassy Wizard as hard as he could, knocking him out.
At the end of the episode the Grassy Wizard was hospitalized and Fern was upset but Jake said he did the right thing. Finn met with the two and told Fern he did a good job (Fern was not yet named so Finn called him "Grass me" while others mostly called him Grass Finn.) Fern then realized that he is not Finn so he came up with a new name, Fern the Human. Fern stole Starchy's motorcycle and took off.
In "The Invitation," it is revealed that Fern lives on top of the Tree Fort roof. Finn was about to leave for his journey to the islands so he wanted Fern to fill in for him while he was gone. Fern was flattered by this but Finn told him to kill monsters his way. It is also revealed that Fern crashed Starchy's motorcycle as well.
In "Whispers," Fern begins to become even more jealous of Finn.
In "Three Buckets," Fern finds out that he can make himself look like Finn, then he traps Finn on the other side of a locked stone door and tells Finn that he was Finn for too long and that it was his turn. After Finn discovers that he can turn his metal arm into rock breaking device, he breaks down the door and turns on weed-whacker mode. He tackles Fern and they fight, until Finn accidentally cuts Fern to pieces.
Essentially, Fern is dead, but a mysterious man picks up his remains and puts it in a bucket and walks off. Fern's eyeball moved implying that he can still be alive in a way.
Fern's original form took on the appearance of a humanoid figure composed of grass.
He later took on the embodiment of Finn the Human. The notable differences compared to Finn however, is that he's entirely made of grass, from his skin to his hair and clothing; he also has blockier proportions, and is barefooted with two pointed toes on each foot.
He originally communicated in gibberish-like words, an example being "Mwah!", but he soon learned how to articulate and hear normally.
As he was recently brought into existence, Fern initially thought of himself as the true Finn, before he began taking on his own personality as Fern. Fern doesn't understand much of what is considered right or wrong in a situation (such as when he suggested severing Sweet P's tendons to stop him from running), and is also considerably more aggressive and prone to violence than Finn, possibly due to the influence of the Grass Demon.
His memories are those of Finn's, so he goes through somewhat of an identity crisis after it's pointed out to him he's not Finn. Fern developed an inferiority complex of sorts after he failed to protect Ooo while Finn was away at sea. Afterwards, in the presence of Finn, he feels like embracing his own personal identity leaves him overshadowed and as an inferior copy of Finn, which leads him to attempt a takeover of Finn's life.
It's worth mentioning, however, that even with this attempt, Fern does not actively seek to harm Finn, instead locking him in an underground tomb with the promise that he'll regularly return with food and water.
Being entirely composed of grass, he has the ability to change his appearance and shapeshift into any form he can think of in order to fight enemies. In "Do No Harm," he alters his hand into a grass sword to slice grass monsters in half with single swift strokes. He also changed them to huge thorny fist-like projections to beat up the Grassy Wizard. The main downside to these abilities is that being a body of grass, he can't eat like a human despite being able to taste, smell, and remember food. In "Whispers" and "Three Buckets", it's revealed he has the ability to disguise himself as the real Finn, similar to how the Grass Sword disguised itself as a natural right arm.
- "Reboot" (debut, not named yet)
- "The Invitation"
- "The Light Cloud" (flashback, cameo)
- "Skyhooks" (as Fun the Human)
- "Bespoken For" (flashback, cameo)
- "Hero Heart" (as Fun the Human)
- "Skyhooks II"
- "The Wild Hunt" (hallucination)
- 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)
- Fern was leaked to be in LEGO Dimensions as a playable character. If one examines a Laser Ability Hint Stone and presses the Switch button (top face button) they can see Fern mentioned. Clicking his name shows Finn's spotlight video, hinting at the fact that he may be a switchable character for Finn.
- 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. This is a reference to a Star Trek episode, The Enemy Within.