Iyu Hoshino, who transforms into Crow Amazon, is a central figure in the second season. She is a sixteen-year-old girl who was originally a cheerful and ordinary student, known for laughing often and being skilled at baking cakes. Her life was brutally ended when her father suddenly transformed into a new type of Amazon during his own birthday celebration, proceeding to kill and devour his entire family. Iyu herself lost her left eye in the attack. The incident was witnessed by the Amazon hunter Mizusawa Haruka, who arrived for the party and was forced to subdue her father. Her body, still alive but critically damaged, was recovered by the anti-Amazon organization 4C. Although she died on the way to receive medical attention, 4C received special permission to use experimental technology to resurrect her as a Sigma-type Amazon, a process that involves implanting Amazon cells into a deceased body to create a controllable living weapon.

As a Sigma-type, Iyu is essentially a reanimated corpse. While she retains fragmented memories of her past life, she is initially devoid of all emotion, unable to feel physical pain, and lacks any sense of self or personal desire. Her personality is defined by a profound emptiness; she wears a perpetual deadpan expression and is completely indifferent to how she is treated, whether as a human or a biological tool. Her sole function is to follow orders from her superior, Kurosaki Takeshi, to hunt and exterminate rogue Amazons. In combat, she acts as both a sensor to confirm targets and a front-line fighter, engaging enemies with brutal efficiency and without hesitation. A peculiar remnant of her humanity surfaces only in rare moments when she unconsciously hums a song that her father used to sing with her.

Iyu's role in the story is deeply intertwined with the protagonist, Chihiro, also known as Amazon Neo. Chihiro is immediately drawn to her because, unlike any other human or Amazon he has encountered, he feels no cannibalistic urge to eat her. He later discovers this is simply because she is biologically dead. Despite this, Chihiro becomes fiercely protective of her, even joining 4C on the condition that she be treated with dignity. He constantly tries to reach the human girl inside the emotionless shell, taking her on outings to an amusement park, buying her cake, and showing her photographs in an attempt to make her more human. His persistence begins to have an effect. Though she remains mostly unresponsive, small cracks appear in her emotional void, such as when she speaks Chihiro's name for the first time or shows a glimmer of desire to recover her lost memories. Her key relationships are defined by this dynamic: with Chihiro, she is both an object of obsession and the catalyst for his own struggle for humanity; with Kurosaki, she is a loyal soldier and a tool to be pointed at enemies; and with figures from her past, like her former classmate Nagase Hiroki, she represents a lost friend they are desperate to save.

The character undergoes a significant, though tragic, development arc. She begins as an unfeeling weapon, but Chihiro's persistent kindness gradually awakens her dormant humanity. This process reaches a critical point when her orders are changed to hunt Chihiro after he goes on a deadly rampage. While she obeys initially, she is unable to deliver the final blow. Her transformation culminates when she witnesses Chihiro being brutally beaten by the veteran Amazon hunter Jin Takayama. Seeing his suffering, her suppressed emotions burst forth, and she defies her programming to save him, asking him if he is in pain. In her final moments, as 4C activates a self-destruct program in her armlet to end her life, she fully regains her sense of self. In a final, heartbreaking display of reclaimed humanity, she tells Chihiro that she is happy before her body finally shuts down permanently.

As Crow Amazon, Iyu possesses notable abilities that make her a formidable fighter. Her transformation is triggered by her Neo Amazons Register, which changes color from blue to red. Her Amazon form is a sleek, black, bird-like creature with a distinctive asymmetrical design, featuring a fan of feathers on her back and head, and sharp talons. A permanent scar of her past is her missing right eye, a detail that carries over from her human death. In combat, she relies on incredible agility, acrobatic kicks, and powerful slashing attacks with her talons. Despite having no combat experience before her death, her Sigma-type biology grants her surprising adeptness in battle, allowing her to perform complex maneuvers. Her primary weakness is a reliance on protein to sustain her Amazon cells, and she cannot feel pain, which allows her to keep fighting even when her body is severely damaged, but also further distances her from her human emotions.