Fern was born when the cursed Grass Sword merged with the broken remains of the Finn Sword, trapping an alternate version of Finn's consciousness alongside a grass demon. When he first emerged, he believed he was the genuine Finn Mertens, only to face immediate rejection and confusion when confronted with the actual Finn and Jake. Because he possessed all of Finn's memories without having lived the experiences himself, he suffered from feeling like a fake copy rather than a real person. A malicious entity inside him slowly poisoned his mind, fueling jealousy and a desire to usurp Finn's life and identity.
