Michiru Sagae is a mysterious young girl who appears before Ai Enma and her companions in the fourth season of the series. She initially has no memory of her identity or her past, presenting herself as an amnesiac revenant who feels drawn to Ai. Her appearance is characterized by short teal hair, jade-green eyes, a beauty mark below her right eye, and a similar mark on her neck. She wears a long-sleeved green dress with white leggings.

Michiru's background is rooted in tragedy. She was the daughter of a man working for an electric company in a small town. The son of a local landlord, influenced by his parents' jealousy, coerced her into following him and two other boys to a pond in the jungle. The boys attempted to drown her, but she managed to free herself, and during the struggle the ground collapsed, causing all three boys to fall in and drown while Michiru alone survived. After the deaths, the townspeople turned hostile toward Michiru and her family, blaming her for the accident. They ostracized the family entirely, refusing to serve them in shops. An adult villager tricked Michiru into entering the landlord's storeroom, where she was locked away for ten days, worsening her mother's health. Though she managed to signal her parents by placing a wind chime outside a window, the villagers discovered the reunion. A villager attacked her father with a crowbar, killing him instantly. The landlord then led the mob in pouring gasoline into the storeroom and setting it ablaze to kill Michiru and her mother. In her final moments, Michiru's rage culminated into a powerful grudge that burned the entire village and all its inhabitants to the ground.

Personality-wise, Michiru is portrayed as having been kind and caring toward her family, much like other hell girls before her. However, her experiences left her with a deep sense of injustice and a questioning nature. When she first encounters Ai, she does not immediately accept the role of Hell Girl. Instead, she questions the morality of Ai's job and reflects on the ugly truths of human nature. She initially flees from Ai and her companions, unwilling to accept her fate. Despite this, she slowly becomes interested in the cases they investigate, eventually showing empathy toward victims and even advising a former target not to make his parents worry.

Her motivations evolve from confusion and resistance to a reluctant acceptance of her destiny. She ultimately agrees to become Ai's successor after becoming attached to a particular case. As the new Hell Girl, she takes on the role with a somewhat more compassionate approach than her predecessor, once telling a former victim that she hopes to meet him in heaven. Her abilities include generating tormenting illusions that reflect the past sins of her victims, similar to Ai, as well as summoning supernatural flames fueled by her deep-seated wrath and tragic memories. After assuming her role, she wears a green floral-pattern kimono decorated with large black and red roses. She is also given a new assistant, Yamawaro, who decides to aid her in her duties.

Key relationships include her interactions with Ai Enma, whom she initially judges and questions, but who helps her realize her fate. She also forms a bond with Yamawaro, who becomes her first companion. Her relationship with her parents, though brief in the narrative, defines her tragic past, while her connection to the townspeople and the landlord's son sets the stage for her transformation into a vengeful spirit and ultimately a Hell Girl.

Her development spans the season as she regains her memories, confronts her past, and moves from a lost, amnesiac child to a purposeful figure who accepts the burden of ferrying souls to hell. While she becomes the second Hell Girl, it is noted that she does not cease being one, seemingly bound to the role eternally because of her actions in a previous season. Her notable abilities, as a Hell Girl, center on her power to damn souls and her more merciful perspective, which adds a nuanced layer to her character.