Hirono Shimizu is a fifteen-year-old student from the third-year class of Shiroiwa Junior High School, who becomes a participant in the lethal program known as the Battle Royale. Before the program, Hirono is established as a member of the group led by the volatile Mitsuko Souma. She is described as a very attractive girl, with an appearance that gives the impression of a university student rather than someone her age, and she stands notably tall for a Japanese girl of fifteen. Her background is steeped in delinquent behavior; Hirono has been involved in shoplifting, recreational drug use, and has even engaged in prostitution. She is also known for intimidating other classmates, including Megumi Eto and Kaori Minami. This lifestyle has created a deep rift between her and her parents, who see her as a lost cause.

Despite her reputation as a troublemaker, Hirono's personality is more nuanced than it first appears. She is proud of being genuine, considering herself honest compared to others who maintain false personas, such as Mayumi Tendo. While she has committed crimes, she never envisioned herself as a killer and is fundamentally opposed to the idea of murdering her classmates. She possesses a level of maturity and perception born from her experiences on the streets, which makes her cynical and untrusting of others' motives, yet she harbors a desire to believe in basic kindness. Her only notable fear is of frogs, a phobia that plays a significant role in her fate.

Her key relationships are defined by her association with Mitsuko Souma's gang, which also includes her friend Yoshimi Yahagi. Hirono's connection with Mitsuko is fraught with tension. She is one of the few students intelligent and perceptive enough to see that Mitsuko cannot be trusted from the very beginning. When she learns of Yoshimi's death, she correctly suspects that Mitsuko is responsible. In the film adaptation, this animosity is heightened by a past incident where Mitsuko stole Hirono's boyfriend. In contrast, her encounter with the protagonist Shuya Nanahara leaves a strong impression on her. When Shuya risks his life to stop a firefight between Hirono and Kaori Minami, and later offers to let her join his group despite her weapon and her "bad girl" reputation, Hirono is genuinely moved and wants to trust his sincerity, even though her instincts advise against it.

Hirono's role in the story is that of a reluctant participant who is forced to confront the contradictions of her own nature. Initially, she tries to avoid the game entirely, but she is drawn into a confrontation with Kaori Minami. In the novel, Hirono kills Minami in self-defense during a shootout. In the manga, while the fight also occurs, she chooses to flee after being shot rather than kill her attacker. Throughout her journey, she suffers from a wound that becomes septic, leading to severe dehydration and fever. This physical weakness drives her most desperate act: searching for water. Her development is tragically cut short, but not before revealing that beneath her hardened exterior is a person who yearns for connection and escape. Her final moments are defined by a powerful desire to live and a poignant, if fleeting, hope for salvation. Ultimately, her ability to survive is limited by her distrust and physical condition, but her notable abilities include her resourcefulness and her skill with her assigned weapon, a Smith & Wesson M10 revolver in the novel and manga, which she uses effectively when her life is directly threatened.

Hirono's fate differs significantly across the various adaptations of Battle Royale. In the novel, while trying to drink water from a bucket, she discovers a frog inside and screams. The scream attracts Toshinori Oda, who attacks her. Hirono shoots him, but he survives due to a bulletproof vest and ultimately strangles her to death. In the manga, she is also attacked by Toshinori Oda at a well. After she shoots him, he feigns death and pushes her into the well. She suffers a concussion and, as the water level rises, hallucinates being rescued by Shuya Nanahara and his group. In reality, she drowns in the well, her last thoughts being that the nightmare has finally ended. In the film version, she does not encounter Shuya or Oda. Instead, she tracks down Mitsuko Souma to confront her over her various betrayals and murders. Hirono manages to get the upper hand but hesitates for too long, allowing Mitsuko to retrieve a stun gun. Mitsuko disarms Hirono and shoots her twice in the back as she tries to flee.