Ruby Hoshino — Oshi no Ko

Before becoming Ruby, she was Sarina Tendouji, a seriously ill girl who died before she could pursue her dream of performing. While hospitalized, Sarina loved the idol Ai Hoshino and formed an affectionate bond with Gorou Amamiya, the doctor who shared her enthusiasm for Ai. Her reincarnation as Ai’s daughter gives the ambition she later expresses as Ruby a history almost nobody around her knows.

Ruby initially does not know that Aqua, who has also been reborn, carries Gorou’s memories. The twins share a remarkable origin without knowing the complete truth about each other, a divide that becomes central to Ruby’s later development.

Ai is murdered by a stalker while the twins are still young. Ruby grows toward an idol career in her mother’s memory, but a later trip to Takachiho to film a B-Komachi music video brings news of Gorou’s death. Learning that one of the people implicated in the killings is still alive, she becomes intent on revenge. Darkened star imagery in her eyes accompanies the stark change in her behavior.

After Ai’s death, Miyako Saito raises Ruby and Aqua and takes on responsibility for Strawberry Productions. Her protection and management give Ruby a family anchor even as the twins respond differently to their loss.

Ruby grows more calculating in her media career. During a controversy surrounding a cosplay segment on a web television show she helps present, she makes use of the publicity and its fallout while pursuing her own goal. This strand of the story complicates the earlier image of Ruby as an uncomplicated, hopeful newcomer.

When Kana Arima faces a potential tabloid scandal, Aqua offers the press information about Ai’s secret motherhood to draw attention away from Kana. That decision publicly exposes the twins’ parentage. Ruby, furious that he has surrendered a secret Ai kept to protect her life as an idol, rejects him for a time.

Ruby earns the role of Ai in the film The 15 Year Lie through a private audition against Akane Kurokawa and Frill Shiranui. The project demands more than resemblance to her mother: as Ruby prepares to portray Ai, she confronts painful memories of her earlier life, including Sarina’s relationship with her own mother.

Aqua and Ruby eventually realize that they were Gorou and Sarina in their previous lives. Ruby regains the trust and affection she once felt for the doctor who encouraged her, now aware that he is also her twin brother. Her long-standing romanticized feelings for Gorou create an uncomfortable tension in the present, particularly as the siblings must portray Ai and Hikaru Kamiki in the film.

As the final conflict approaches, Ruby becomes the intended target of an attack by Nino, a former member of the original B-Komachi. Akane Kurokawa impersonates Ruby and is stabbed in her place, but protective precautions allow Akane to survive. Ruby remains alive as Aqua proceeds toward his final confrontation with their father, Hikaru Kamiki.

During the confrontation, Aqua injures himself and goes into the sea with Kamiki; neither survives. Aqua’s choice is bound up with his determination to protect Ruby’s future, but it also leaves her grieving the brother she has only recently recognized as Gorou. The manga depicts no subsequent resurrection or new reincarnation for Aqua.

Ruby struggles deeply after Aqua’s death and withdraws from ordinary activity. She eventually returns to school and to B-Komachi, and the group reaches Tokyo Dome—a stage Ai never performed on. The closing chapter does not present the grief as erased: before leaving for rehearsal, Ruby pays her respects to Ai and Aqua, carrying their memories into the work she chooses to continue.

The collected final volume adds an epilogue told through a later video by MEMcho, who has left B-Komachi and become a producer. Ruby is still active as an idol in that later glimpse. The extra pages give the characters an extended professional aftermath, but do not establish a detailed final outcome for every aspect of Ruby’s personal life.