While Oshi no Ko is fun to watch due to the antics displayed by Aqua and Ruby Hoshino, one must not forget that they are the reincarnations of Ai's deceased fans. If viewers watch the anime with that fact in mind, their perspective of the Hoshino twins, especially Aqua, is bound to change.
Oshi no Ko, written by Aka Akasaka and illustrated by Mego Yokoyari, follows the story of Aqua and Ruby Hoshino, the twin children of pop idol Ai. However, the twist is that both children were Ai's fans in their previous lives and got reincarnated.
Disclaimer: This article reflects the author's views and contains spoilers from the Oshi no Ko manga.
Oshi no Ko: Why Aqua is overly obsessed with his mother Ai?
While both Aqua and Ruby were fans of Ai in their previous life and happened to get reincarnated as her children, only Aqua has shown signs of being overly obsessed with his mother, Ai.
In the meantime, Ruby, who in her previous life as Sarina idolized Ai and wanted to become just like her, isn't that obsessed with the matter. While she is upset about her mother's death, she is focused on using the gifts given to her by Ai and trying to become an idol, just like she wished in her previous life.
So, why is Aqua disturbingly obsessed over Ai Hoshino while Ruby lives a normal life? Unlike Aqua, who in his previous life as Gorou Amamiya had met Ai in person, Ruby, as Sarina, was only a fan of Ai's and idolized her through television. In addition, she had gone to B-Komachii's concert only once in her lifetime.
In his previous life as Gorou Amamiya, Aqua was Ai's doctor and had spent a lot of time alongside her during her pregnancy. Thus, Aqua was already close to Ai in his previous life and had seemingly developed feelings for her.
This in itself is a disturbing precedent considering that Gorou was in love with Ai while Aqua is her son. This means that Aqua, due to his mind being that of Gorou, is obsessed with his mother, Ai, because of his romantic feelings for her.
Having such strong feelings for his mother, Aqua could not handle the pain of witnessing her death at the hands of the disgruntled fan Ryosuke. So, right after he lost his mother, Aqua started plotting revenge against his father, who was responsible for getting Ai killed by Ryosuke.
Having followed his goal for nearly eight years now, it seems impossible for Aqua not to be obsessed over his mother, given that he is yet to identify his father in the anime.
As for the manga, Aqua's mental health seems to be worse, as he has not just identified his father, but he also plans to bring his actions known to the public and possibly kill him in the future. During his revenge journey, he has already betrayed Ruby and Akane's trust.