Mizuho Inada is a third-year student at Shiroiwa Junior High School, assigned to class 3-B. She is known among her classmates for her vivid imagination, spending much of her time living in a fantasy world where she believes she is a warrior named Prexia Dikianne Mizuho, sent by the god Ahura Mazda. She carries a pendant that she believes is a mystical quartz containing the god, though it is actually a glass trinket she bought from a mail-order catalogue. In the manga version, her delusions center around a holy trinity of gods, and she hears voices that she interprets as their commands. She often involves her close friend Kaori Minami in her fantasy, referring to Kaori as Lorela Lausasse Kaori, and also includes Megumi Eto, calling her Meriai Lektra Megumi. Her profile in the manga states that she suffers from undiagnosed paranoid schizophrenia and delusions.

When the Battle Royale program begins, Mizuho is the first girl called to receive her equipment, and she leaves the classroom crying and embracing Kaori. On the island, the stress of the game accelerates her mental breakdown, and she begins to believe that her fantasy world has become real. She interprets the deaths of her friends as the work of demons, which she associates with the more dangerous participants, such as Kazuo Kiriyama and Mitsuko Souma. In the novel, she survives until the later hours of the program. After witnessing Kiriyama kill Mitsuko, she becomes convinced that Ahura Mazda wants her to kill Kiriyama. She creeps up behind him with her designated weapon, a double-bladed knife that she imagines as a great sword, but Kiriyama shoots her in the chest and face without even turning to look at her. In the manga, she encounters Shuya Nanahara after he leaves the lighthouse and attempts to kill him, but he escapes. She then constructs an elaborate shrine to her god and is killed by Kiriyama while worshipping there. In the film adaptation, Mizuho and Kaori manage to reunite, but they later die together; a report states they killed each other over a life preserver, with Mizuho's knife in Kaori's back and Kaori's ice axe in Mizuho's stomach.

Mizuho's role in the story illustrates the psychological disintegration that the program can inflict on participants who are already vulnerable. Her fantasy initially serves as an escape from reality, but the trauma of the game pulls her deeper into a state where she cannot distinguish between imagination and truth. Her key relationships are with Kaori Minami, whom she involves in her delusions, and Megumi Eto, who tolerates her eccentricities. She has no notable combat abilities and relies entirely on her assigned weapon. Her development is tragic: she begins as a harmless, dreamy girl and ends as a psychotic believer whose hallucinations lead to her death.