Mistilteinn is a dream demon born from the space between nightmares, a realm shaped by accumulated human negative emotions such as hatred and spite. This origin grants her exceptional power, acknowledged by figures including Pharos Hercules. Her appearance features snow-white hair adorned with a red vine crown, teal eyes with vine-shaped pupils, and a dress formed from red and black plant-like material. Two vine-like tendrils extend from her back, serving as her primary weapons.

She exhibits a vicious and sadistic personality, toying with victims to demonstrate superiority before killing them. Her actions are driven by a desire to consume other dream demons, whose deaths "fill her stomach," and she takes pleasure in destroying human ambitions and dreams. Despite this ferocity, she displays cowardice when threatened, resorting to tactics like taking hostages to avoid direct confrontation. Her compatibility with the human Ryōta Ijima stems from their shared inclination to crush dreams; he devises plans to implant dream demons in students, while she executes them, turning victims into listless shells.

Her history includes indiscriminately slaughtering dream demons in the dream world, notably killing allies of the dream demon Lestion. After crossing into the human world, she partners with Ijima, who becomes her vessel. Their scheme involves forcing dream demons into students, then destroying those demons once the students reveal their aspirations, extinguishing their ambitions. Key victims include Yumi’s dream demon Serio and Isana Tachibana’s dream demon Palette. During Isana’s ordeal, Mistilteinn traps her in a water orb to lure out Palette, though this fails. She later engages Merry Nightmare and Yumeji Fujiwara, taunting and overpowering them before temporarily retreating.

In her final battle within Engi’s daydream, Mistilteinn overpowers Engi Three-Piece by impaling her with a thorn and mortally wounds Lestion. After Lestion sacrifices himself to create a "dream-depleting" bullet, Chizuru Kawanami fires it, hollowing Mistilteinn’s heart without killing her. She then traps Merry in the "Bud of Despair," a flower amplifying sorrow. When Yumeji confronts her with John Doe’s guillotine-saw, his resilience exposes her weakness: strength in dreams derives from heart, not physical power. After Merry breaks free, Mistilteinn swallows Palette as a shield. Merry disintegrates half her body by striking the bullet-weakened area, freeing Palette and causing severe injury. Engi ultimately slices Mistilteinn in half, dissolving her into red vines and leaves.

Her abilities surpass typical dream demons, leveraging plant-based and water manipulation. She creates water-based projectiles, expands droplets to drown victims, and wields vine tendrils for piercing attacks. She also summons spear-like weapons from crimson petals, seeds that grow into despair-inducing flowers, and thorn projectiles. A defensive ability allows her to swallow dream demons whole, using them as leverage against attackers. Her combat prowess includes effortlessly blocking strikes, shattering weapons, and outpacing combined assaults.

This character is exclusive to the anime adaptation and does not appear in manga continuations, spin-offs, or other media.