Grass Fern

At first, he talked in strange nonsensical words like "Mwah!" but then he learned to talk and listen normally. When he was first created, he thought he was the real Finn, but eventually he became his own person called Fern. Fern doesn't really understand what is right or wrong in certain situations (like when he suggested hurting Sweet P to stop him from running). He is also more aggressive and violent than Finn, maybe because of the influence of the Grass Demon. Fern has Finn's memories, so it's confusing for him when people say he's not really Finn. Fern feels inferior because he couldn't protect their land when Finn was away. When Fern is around Finn, he feels like his own identity is not as important and tries to take over Finn's life. But it's important to note that Fern doesn't want to hurt Finn, he just wants to take his place for a while.

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.