Mari Igarashi is a supporting character who appears in the episode titled The Sandman's Lair, which is part of the anthology series. She is a close friend and fellow writer of the story's main protagonist, Yuji Hirano. Her background is that of a writer, and she initially interacts with Yuji during a casual meeting where she notices his exhausted appearance and asks about his well-being. Mari is characterized by her concern for Yuji and her initial skepticism toward his extraordinary claims. When Yuji explains that he has stopped sleeping because his dream self has become aware of reality and is trying to swap places with him, Mari responds with disbelief and suggests that he see a doctor, attributing his condition to stress over his writing career. Despite her doubts, her motivation is rooted in genuine care for her friend, and she agrees to visit his home to witness the phenomenon for herself. Her role in the story is that of an ally who attempts to help Yuji manage his terrifying condition. As the situation escalates, Mari takes increasingly drastic measures to keep Yuji awake and prevent his dream self from taking over, including binding his wrists and ankles with tape at night and preventing him from falling asleep. However, her compassion leads her to unbind him out of pity, which allows the dream self to begin emerging. Upon witnessing Yuji's arm emerging from his own mouth, she is shocked but still tries to support him. Her development shifts from a skeptical observer to someone who becomes deeply entangled in the horror. In a final effort to anchor Yuji to reality, Mari ties herself to him, but she is ultimately pulled into his body and absorbed, becoming part of his composite self. She does not possess any supernatural abilities; her notable traits are her loyalty, resourcefulness in trying to restrain Yuji, and her gradual descent from disbelief to helpless involvement in the nightmare. Her disappearance leads to a police investigation, but her fate is sealed within Yuji's fractured identity.