Mephisto, operating under the alias Dr. Shinigami and later revealed as Shimura, belongs to the same ancestral lineage of Dream Guardians as Rem Ayanokōji. He wields the psychic ability to enter and manipulate dreams, but corrupts this legacy for malevolence. While Rem safeguards humans from dream-invading demons, Mephisto invades victims' dreams to murder them, perverting his inherited powers into an offensive weapon.

In "New Dream Hunter Rem: Massacre in the Phantasmic Labyrinth," Mephisto expands his targets to include artificially created beings. He seizes control of Julian, a genetically engineered human crafted by Victor Frankenstein, by bewitching him within the dream world. This transforms Julian into a monstrous entity, weaponizing the dreamscape to corrupt and manipulate life. Mephisto exploits the boundary between dreams and reality to inflict harm, confronting Rem in the labyrinthine dream world as she battles to free Julian.

Consistently depicted as Rem’s dark mirror, Mephisto embodies the corrupted potential of their lineage. His methods rely on psychological manipulation and dream-based violence to fulfill destructive ambitions.