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 Finn sword 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 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 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, especially 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 then walked in and said hello to BMO. Confused by this BMO ran into a hole where Fern wouldn’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 harm 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 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 weedwhacker mode. He tackles Fern and they fight, until Finn accidentally cuts Fern to pieces.
Essentially, Fern is not alive, 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...