The following article contains spoilers for Oshi no Ko. The latest chapter is available on Manga Plus.The anime adaptation is currently streaming on HIDIVE.
Oshi no Ko is Spring 2023's most critically acclaimed anime by far, telling a thriller mystery in the form of a deconstruction of the idol industry - and entertainment as a whole. After our main character's mother is murdered, Aquamarine Hoshino is determined to find and kill the true perpetrator: his father.
Moreover, his motivation for revenge may not just be his past self's desire to see Ai brought to justice, but a newly inherited trait from his father in Aqua's new body.
The Compassionate Doctor
During Goro's backstory, we find out that his mother died during childbirth, and so he grew up without parental figures aside from a strained relationship with his grandparents. As a result, he made the resolve to enter the medical field, and become a gynecologist to ensure that no other mothers would die during childbirth. Upon reincarnating, Goro (now Aqua) mostly retained his traits and knowledge from his previous life. However, his body is still that of a child's, finding himself needing to sleep for long periods of time. Later on in the manga, after he reaches adolescence, it's stated that Aqua has trouble recalling memories of his years as a baby, due to infantile amnesia. His memories may be intact, but his physical - and perhaps even mental condition, is that of a different person. The experiences we have growing up shape who we are as people, but what if there's more to it?
In chapter 95 of the manga, Aqua tells Ichigo, his parental guardian that he's given up on revenge, after finding out that his "father" is already dead. Little does he know however, that his real father is still alive and out there. Upon this realization, Aqua is in denial about it and even wishes that there was some way he could live for anything other than vengeance. Moreover, in an earlier chapter, Aqua's consciousness takes the form of a shadowy Goro, telling him that he's not allowed to move on, implying that Aqua doesn't necessarily want revenge, but feels like he has to.
The Sins of the Father
One of Oshi no Ko's underlying themes is destiny. It's stated multiple times how much Ruby resembles Ai in her personality and ambition to be an Idol, just like her mother. And for Aqua to pursue a career in acting - just like his father did. In chapter 96, a new character by the name of Hikaru Kamiki is introduced and is revealed to be Ai's former lover - and the father of her children. It's all but confirmed that he is indeed a murderer with a specific target for female actresses - which Ai had been, at the time of her death. Upon Akane's research into who he actually was, we realize that during Hikaru's childhood as a student at an acting school, one of the teachers assaulted him and became pregnant with his child - who would become Aqua and Ruby's half brother, Taiki Himekawa. Years later, after the latter's birth, Himekawa's legal father would commit a murder suicide towards his wife. It's strongly implied that Hikaru may have had something to do with it, as a form of revenge. What's more, at the beginning of the story, Aqua deduces that Ai's killer was manipulated, so Hikaru wouldn't get his hands dirty.
During his introduction, he is seen murdering a famous actress, albeit discreetly. Aqua demonstrates exceptional skill with networking and getting people to do what he wants - a tactic very similar to what his father uses. There is a pattern happening here as well, because of Hikaru's trauma, he targets and kills all those who match the same description as his attacker. Revenge is ingrained in his reason for living - similar to Aqua. Thus, Ai's twins may have a lot more in common with their father than just appearance.
The Sickly Idol
Ruby, being a terminally ill patient in her past life, dreamt of becoming an idol, and so once she was (re)born, she made it a goal to achieve that. It could be argued that her happy-go-lucky nature is because she's unaware that Ai's killer is still out there, and it may be true, as her discovery of Goro's body only saddens her, rather than leads her to speculate who the actual killer was. And as we saw during Ai's death, she seemingly forgave the killer and smiled until the end; so these could be traits that Ruby inherited. But yet when she finds out there was indeed an accomplice, who orchestrated both Goro's and Ai's death, Ruby too, desires vengeance.
Lying or Acting?
During Goro's life, he lives as an honest man, and so any attempts to lie, always fall flat - a very different trait from Aqua, who skillfully lies his way through life; something both Ai and Hikaru do well. In Ruby's case, she began with a desire to live honestly, using Goro as an inspiration, but upon claiming her desire for vengeance, she too begins to lie. We can see this one of two ways: the siblings have always been like this, or the loss of "innocence" triggered something within them. This can further be backed up when in the manga, Aqua states that he has begun to lose his self as Goro. Though this brings into question, who is the Goro-like silhouette that frequently taunts him?