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 Finn sword and the Grass Demon expressed desire to get out. The Grass Demon told the Finn Sword 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 then 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 then 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.
In "Seventeen," a mysterious green knight crashes Finn's birthday party. In a match of arm-wrestling it is revealed that this green knight is Fern. Fern has been revived by the mystery man, who is revealed to be Princess Bubblegum's Uncle Gumbald. He is working for Gumbald along with Lolly and Chicle.
In the series finale "Come Along With Me," Finn and Jake, wanting to stop the Gum War, unleashed the nightmare juice given to them by Nightmare Princess, forcing them, Fern, Princess Bubblegum, and Gumbald into a hallucinatory slumber. In the dream, Fern transformed into a bird/pterodactyl-like and flew away, while Finn decided to follow him after turning into a butterfly. Finn tried to calm Fern down, and two plummeted to the ground. Finn attempted to appeal to Fern by reminding him that they are "two sides of the same coin" and that they share the same torment. However, Finn and Fern continued to fight, but were able to find common ground after Jake brought them Finn's vault. Together they went in to confront the Grass Demon. Fern killed it, ridding himself of its influence once and for all. However, with no Grass Demon to control him, Fern began to deteriorate, as he can have no body without the Grass Demon.
After waking up, the Gum War was called off but GOLB came leading to a different battle. Fern looked as he did before becoming the Green Knight but was beginning to disintegrate, but nevertheless wanted to fight with the others against GOLB. GOLB was later defeated, but Fern was still disintegrating, nearly dead. He told Finn his dying wish, to be planted at the Tree Fort (which unbeknownst to him, had been destroyed). Fern died, and what remained was a small seedling in the shape of the Finn Sword. That night, Finn and Jake planted the seedling together at what remained of their home. The seeingling immedatley grew into new tree to sprout, with a new Finn Sword growing inside it.
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.
In "Seventeen," Fern took on a completely different look after Gumbald had genetically modified him. Fern's new body is much larger than his old one. It's now darker shade of green; his waist is darker than the rest of his body. He has thorns sticking out from his limbs and head. He usually has his right arm morphed into the shape of a blade. Fern still maintained having two toes on each foot. His head also has multiple holes scattered across his face, but he can revert it to his original face as he pleases.
In "Come Along With Me," after removing the Grass Demon from him in the shared nightmare juice dream, Fern reverted to his original appearance but began to disintegrate as he could have no body without the Grass Demon. Fern later disintegrated leaving only a seedling in the shape of the Finn Sword.
Finn and Jake planted Fern's seedling at the site of the destroyed Tree Fort. The seedling immateriality grew into a tree resembling the tree fort with a Finn Sword growing out of it. 1000 years later the Fern tree still remained, growing to a massive size, with the Finn Sword remaining until being found by Shermy and Beth.
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. Afterward, 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.
As the Green Knight, Fern lost his desire to be just like Finn, but now he sees himself as a better version of Finn. Despite this, he still thinks of Finn's memories as his own. He also developed a desire to kill Finn, but each time he attempts to do so either Aunt Lolly or Uncle Gumbald stops him before he gets a chance to cause any critical damage to him. Overall, he has become much more confident in his abilities.
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; in addition, Fern is shown to be able to photosynthesize with light. In "Seventeen," Fern's new form as the Green Knight allows him to produce other plants, morph his arm into a blade, sprout multiple tentacles from his body and reveal his face depending on if he wants to or not. He can also form an axe from his tentacles which can turn into a snake controlled by him. The Green Knight can still choose to use the grass sword regardless of already having an axe and a blade shaped arm.
- "Two Swords" (not named yet)
- "Do No Harm"
- "Three Buckets"
- "Seventeen" (as the Green Knight)
- "Come Along With Me"
- "Lemonhope Part 2" (unofficial debut, tree)
- "Graybles 1000+" (tree)
- "Reboot" (official 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, cameo as the Green Knight)
- "Temple of Mars" (as a Frog)
- "Gumbaldia" (as the Green Knight)
- 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.
- After Fern's seedlings grows up, it resembles a willow tree, just like the original Tree Fort, but willow tree is not a fern.
- Strangely, despite being able to talk, Fern is unable to breathe, eat, or play the flute. He can, however, create his own "food" by Photosynthesis just like a normal plant.
- In the episode "Three Buckets," Fern wanted to be the only Finn and take over his life. This is a reference to the Star Trek episode The Enemy Within.
- His armor as the Green Knight resembles Alex Mercer's armor from the Prototype video game series.
- His title and challenge towards Finn as the Green Knight are a reference to the Green Knight of Arthurian legend, who challenged the knights of King Arthur to strike him with his axe, on the condition that he returns the following year to strike them back. Much like in the episode, the Green Knight survives being decapitated.
- Fern's story is similar to that of Anakin Skywalker/Darth Vader from the Star Wars saga.
- The two both committed questionable violent actions, Fern beat up the Grassy Wizard while Anakin killed a tribe of Tusken Raiders.
- Both chose become evil and took on new personas. Fern became the Green Knight, wihile Anakin became Darth Vader.
- After becoming evil, both were nearly killed in a fight by one who once their friend. Fern was nearly killed by Finn and blown into pieces of grass, while Obi-Wan Kenobi chopped off Vader's legs and left him to burn in lava.
- After their repetitive fights the two were each retrieved by their new masters and were given "modifications." Fern was retrieved by Gumbald and was modified becoming more powerful as the Green Knight, while Emperor Palpatine placed Darth Vader in life-supporting suit of armor.
- Both eventually redeemed themselves but had to give up their lives in the process. Fern removed the Grass Demon controlling him, but could no longer exist without it, while Vader sacrificed himself and killed Palpatine but had to get shocked to death.
Appearances in other media
- Main article: References in other media
- Fern is a playable character in the 2015 video game LEGO Dimensions. In order to unlock him, Finn must go in The Badlands and interact with the Grass Sword statue in the Land of Ooo during the open world mode, Fern will pop out of the ground and it is possible to swap Finn and Fern from the character select wheel.