Oshi no Ko Reincarnation Explained: Who Were Aqua and Ruby Before?

Aqua and Ruby Hoshino are both reincarnated people in Oshi no Ko. Aqua was previously Gorou Amamiya, the doctor who cared for the pregnant idol Ai Hoshino. Ruby was previously Sarina Tendouji, a terminally ill girl who knew Gorou and idolized Ai. Both died before being reborn as Ai's twin children, while retaining memories of their previous lives.

That sounds like the setup for a conventional reincarnation story, but Oshi no Ko uses it differently. The series is far less interested in explaining a supernatural reincarnation system than in showing how a second life changes two people who carry unfinished hopes, grief and relationships from their first lives.

Spoiler warning: This article discusses major anime-confirmed developments through Oshi no Ko Season 3, including the identities of Aqua and Ruby's previous lives.

Who was Aqua before reincarnation?

Before becoming Aquamarine Hoshino, Aqua was Gorou Amamiya, an obstetrician working in the countryside. His connection to Ai begins when the pregnant idol unexpectedly arrives at his hospital. Gorou is already an Ai fan, but he also has a professional responsibility to protect her pregnancy and keep it secret.

Gorou never gets to see that responsibility through. On the night Ai is due to give birth, he encounters a stalker and is murdered. He then awakens as one of Ai's newborn twins, Aqua, with his adult memories and medical knowledge intact.

The reincarnation gives Gorou something he could never have expected: the chance to live as the child of the idol he admired. For a short period, that second life is almost absurdly fortunate. After Ai's murder, however, Aqua's memories of being Gorou become part of what drives his obsession with finding the person responsible.

Who was Ruby before reincarnation?

Ruby's previous identity was Sarina Tendouji. Sarina was a young patient suffering from a serious illness, and Gorou knew her before the events surrounding Ai's pregnancy. Her physical condition limited the life she could live, but she found joy in idols, especially Ai Hoshino.

Sarina dreamed of becoming an idol herself. She died at only 12, never getting the opportunity to pursue that dream. Reincarnation as Ruby therefore means something very different for her than it does for Aqua. Ruby receives a healthy body, a famous idol for a mother and a genuine second chance at the future Sarina wanted.

This is why Ruby's determination to enter the entertainment industry is more than simple admiration for Ai. Becoming an idol is a continuation of a dream that began before Ruby Hoshino existed.

Were Gorou and Sarina connected before becoming twins?

Yes. Gorou and Sarina knew each other in their previous lives. Their relationship as doctor and patient is one of the reasons their reincarnation as siblings carries so much emotional weight.

Sarina's enthusiasm for Ai strongly influenced Gorou. What initially looks like two separate reincarnation stories is therefore a loop: Sarina admired Ai, Gorou was affected by Sarina's passion, both died, and both were ultimately reborn as Ai's children.

The coincidence is intentionally extraordinary, but the anime has not established a detailed rulebook explaining why these two people specifically were reincarnated together. Oshi no Ko treats the event as a central mystery and narrative premise rather than presenting a game-like reincarnation mechanism.

Do Aqua and Ruby remember their past lives?

Yes. Both retain substantial memories of who they were. Even as babies, they think and communicate with awareness far beyond their physical age. Those memories continue to influence their goals as they grow older.

Their retained memories do not mean they remain exactly the same people forever. Aqua and Ruby accumulate new relationships, experiences and trauma in their second lives. The longer they live as the Hoshino twins, the more their old and new identities become intertwined.

Aqua's medical background contributes to his unusually mature thinking, but his life as Aqua changes him profoundly. Ruby likewise carries Sarina's dream forward while developing an identity, career and relationships that belong specifically to Ruby.

Do Aqua and Ruby know who the other person used to be?

Not at first. The twins quickly realize that each of them is reincarnated, but they do not immediately reveal their former names. That creates one of the story's longest-running pieces of dramatic irony: the audience understands the connection between Gorou and Sarina while Aqua and Ruby spend much of their new lives unaware that the person they once knew is standing beside them.

Season 3 finally brings that connection to the surface. Crunchyroll's official coverage of the Season 3 finale specifically describes the emotional moment as Gorou and Sarina reconnecting as Aqua and Ruby, and the commemorative visual overlays their previous identities behind the twins.

The reveal matters because Ruby's memories of Gorou are deeply important to her, just as Aqua carries memories of Sarina. Learning the truth changes how both characters understand the strange second life they have shared.

Why were Aqua and Ruby reincarnated as Ai's children?

The anime has not given a simple mechanical explanation such as a named power that selected Gorou and Sarina. What it does establish is the thematic connection between all three characters.

Sarina loved Ai and wanted the life of an idol. Gorou's admiration for Ai was connected to Sarina. Rebirth as Ai's twins gives both of them an impossible second chance, but the story then takes that apparent miracle away from them when Ai is killed.

That contrast is essential to Oshi no Ko. Reincarnation does not erase tragedy. Aqua and Ruby get new lives, yet they remain shaped by deaths from both of their identities. The supernatural premise becomes a way to examine memory, ambition, grief and the identities people construct in the entertainment industry.

How reincarnation shapes Aqua

Aqua's second life is dominated by investigation. Because Gorou was murdered shortly before Ai gave birth and Ai was later killed by a stalker, Aqua becomes convinced that the events are connected to someone inside the entertainment world.

His adult memories make it easier for him to manipulate situations and think strategically, but they also prevent him from experiencing childhood normally. Aqua is repeatedly caught between living his new life and treating that life as a vehicle for resolving the deaths that came before it.

How reincarnation shapes Ruby

Ruby initially represents the hopeful side of reincarnation. Sarina could not dance, perform or grow into adulthood, while Ruby has the physical health and family connection needed to chase the idol dream she once watched from a hospital room.

But Ruby's past is not simply inspirational. Her attachment to Gorou and her memories of her first life become increasingly important as the story progresses. Season 3 makes those unresolved emotions impossible to separate from her current identity.

Is Oshi no Ko an isekai?

Not in the usual sense. Aqua and Ruby are reincarnated, but they are reborn in the same modern world rather than transported to a fantasy world. Oshi no Ko uses reincarnation as its supernatural hook while focusing primarily on Japan's entertainment industry, family, celebrity culture and revenge mystery.

Calling it a reincarnation drama is more useful than expecting the conventions of a typical fantasy isekai. The story rarely treats reincarnation as a power system. Its importance comes from what Aqua and Ruby remember and how those memories affect their choices.

Why the Season 3 reveal changes the story

For much of Oshi no Ko, the twins' past identities function as information they keep even from each other. Season 3 removes that barrier. The Gorou-Sarina connection is no longer only background information for viewers; it becomes something Aqua and Ruby must emotionally confront themselves.

That is particularly important heading toward Oshi no Ko's fourth and final season, which is officially in production. The final season now inherits characters who understand far more about the person beside them than they did at the beginning of the story.

The simple answer

Aqua was Gorou Amamiya and Ruby was Sarina Tendouji. Gorou was Sarina's doctor, Sarina was a devoted Ai fan, and both died before being reincarnated as Ai Hoshino's twins. They retain memories of those lives, but initially do not know the other's former identity.

Oshi no Ko never needs reincarnation to become a complicated supernatural system. Its real purpose is emotional. Aqua and Ruby receive the second lives they could never have imagined, then spend those lives dealing with dreams and tragedies that began before either of them was born as a Hoshino.