Mistleteinn Treesea

Mistleteinn is a malevolent and vicious Dream Demon who appears as an anime-exclusive villain in the animated adaptation of Dream Eater Merry (or Yumekui Meri in Japanese version). She has the power to force Dream Demons into human vessels, later targeting and killing them, resulting in the vessel losing all ambition in their goals. She is extremely strong, as it is noted that she is acknowledged by Pharos Hercules for her power.

Personality
Despite having the outward appearance of a sweet little girl, Mistleteinn is a vicious, monstrous and intelligent sadist. She is playful in the sense that she enjoys toying with her victims almost as if she was purposely proving that she is a lot stronger and better than her victims before killing them.

Mistleteinn appears to be sustained by killing other dream demons as she hopes that their deaths will fill her stomach. In light of this, she does not feel guilt upon destroying a child's dreams and ambitions; in fact, her facial expressions and outbursts indicate that she is aware of what she is doing and enjoys it, further highlighting her sadistic tendencies. In spite of displayed savagery, she has a cowardly side as she was willing to use dirty tricks to prevent her enemies from attacking her (as seen in the final episode when she swallowed Pallette and used her as a hostage in order to discourage Merry and Engi from attacking her).

Mistleteinn does form a bit of a relationship with the equally sadistic Ryota Ijima. But the relationship that they have is pretty much limited to them praising and admiring each other's sadism. Other than that, she rarely interacts with Ryota that much, and only Ryota shows a bit of a care towards her when she got shot by Chizuru and Lestion.