Fern.

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 the entirety of Elements, he has green and white striped arms and legs, wears dark green pants with light-green buttons, a light-green shirt, his head is a green peppermint and the two points on his hat are candy wrappers. 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. Personality 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. 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. 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 resemble Finn in grass form, even gaining the ability to speak. He then revealed that he believed that he was Finn, considering the real Finn a threat and, later, an imitator/fan. When Jake confirmed the real Finn's identity, Fern became upset and ran from the duo.

Still believing himself to be Finn, Fern traveled to Finn and Jake's tree fort, greeting a confused BMO, who panicked and hid. After looking through a scrapbook of Finn and Jake's adventures, Fern became frustrated and confused and began to trash the fort. A frightened BMO called Finn and Jake and the two came straight home. Jake was furious with Fern for scaring BMO and wrecking 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. At the top of the mountain they saw the Grassy Wizard, who recognized Fern. He then deduced that he came from the Grass Sword, leading Fern to realize that the wizard was his creator. Fern then proceeded to ask why he (as the grass-sword) existed, only to learn he only made the Grass Sword 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 assured him that 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 came to terms with his identity, so he coined a new name, Fern the Human. He then 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 due to his inabillity to do things the right way. He later thinks up a plan to become "The only Finn".

In "Three Buckets," Fern finds out that he can make himself look like Finn. 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 a 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 recovers his remains, puts it in a bucket and walks off. Fern's eyeball moves, 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 the 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 themselves, Fern, Princess Bubblegum, and Gumbald into a hallucinatory slumber. In the dream, Fern transformed into a bird/pterodactyl-like creature and flew away, while Finn decided to follow him after turning into a butterfly. Finn tried to calm Fern down, and the two plummeted to the ground. Finn continued to try and appeal to Fern by reminding him that they were "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 to 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.