Aqua Hoshino in his youth is defined by a profound and tragic transformation, shifting from a remarkably gifted child enjoying a peaceful life to a young boy consumed by a singular, dark purpose. His character is inseparable from his unique origin, as he is the reincarnation of Gorou Amamiya, a thirty-year-old obstetrician and devoted fan of the idol Ai Hoshino. After being murdered by a deranged stalker, Gorou is reborn as Ai's son, retaining all the memories and maturity of his past life.

In his earliest years, Aqua displays the hallmark traits of his past life manifesting in a child's body. He is observant, exceptionally intelligent for his age, and capable of a level of detached analysis that surprises the adults around him. He enjoys a relatively carefree existence, basking in the simple joys of being the child of his favorite idol. However, he is also keenly aware of the need to maintain appearances, carefully hiding his past-life memories and his true identity as Ai's son from the public. During this period, he accompanies his mother to a film set where he meets director Taishi Gotanda. Recognizing the boy's unusual intellect, Gotanda becomes a mentor, and Aqua begins training as an actor, though he often feels his performances are stiff and overly calculated.

This peaceful childhood is brutally shattered at the age of four when Aqua witnesses his mother being fatally stabbed by a stalker at their home's front door. This singular event is the catalyst that rewires his entire personality. The trauma of seeing Ai's murder and feeling her body grow cold in his arms instills a deep-seated post-traumatic stress disorder and extinguishes the innocence of his early years. His personality undergoes a dramatic and permanent dark turn. The once relatively normal and happy child becomes cold, calculating, astute, and intensely secretive.

His core motivation from this point forward is a relentless and unwavering quest for revenge. He quickly deduces that the stalker was not acting alone and must have been given Ai's location by someone close to her. Aqua concludes that this person is his and his twin sister Ruby's unknown biological father, a man he believes is responsible for the murder. He swears a private vow to find this man and kill him, and everything he does from that moment is a calculated step toward this goal. This revenge mission becomes the central pillar of his identity, subsuming any other potential ambitions or desires.

Within the story, young Aqua's role is that of the tragic, driven protagonist. He is not a hero in the traditional sense but a child maneuvering through the dark underbelly of the entertainment industry, which he sees as a necessary battlefield. He re-enters acting not out of passion for the craft but as a means to gain power, influence, and proximity to those who might have known his father. He uses every opportunity to gather intel, suspecting various industry figures as potential candidates for his father. For instance, he uses a role in a live-action drama to get closer to a producer who knew Ai. He is also fiercely protective of Ruby, initially trying to sabotage her dream of becoming an idol out of a desperate fear that she will suffer the same fate as their mother.

His most significant relationships are defined by his trauma. With his twin sister Ruby, he is overprotective and secretive, keeping her in the dark about his true motives while trying to shield her from the world that killed their mother. His relationship with Kana Arima, a childhood acquaintance and talented actress, is complicated; he respects her skills and even uses her connections to advance his search, but his emotional walls prevent any genuine closeness. His bond with his mentor, Taishi Gotanda, is one of the few where he can be more honest about his struggles as an actor, seeking to learn "emotional acting" to better achieve his hidden objectives.

Aqua's development as a young character is less about linear growth and more about the deepening of his obsession. He becomes an apprentice to director Gotanda to hone skills that will be useful for his revenge. He learns to be a masterful manipulator, perfectly playing roles on reality dating shows not for fame but to gather information and cultivate useful allies. Despite his cold exterior, glimpses of his buried humanity surface in moments of genuine care for those close to him, such as when he saves Akane Kurokawa from a suicide attempt, driven by disgust at the toxic nature of the industry and her exploitation. However, he immediately re-contextualizes this act, seeing Akane's ability to mimic his mother's persona as an asset that makes her "indispensable for his goal".

Notable abilities possessed by the young Aqua stem directly from his dual nature as a child with an adult's mind. He possesses the strategic intellect and patience of a thirty-year-old man, allowing him to plan complex, long-term schemes. He is a gifted actor, though he personally feels his performances are too calculated and analytical, a flaw he actively seeks to overcome by learning to channel genuine emotion. His most chilling ability is his capacity for manipulation and emotional detachment, allowing him to feign emotions, form strategic relationships, and threaten adults without hesitation, all in service of his revenge. He is also highly observant and a capable strategist, able to analyze situations and people to exploit their weaknesses for his own ends, such as when he masterminded a social media campaign to restore Akane's public image. The distinctive star-shaped pupil in his right eye, a trait inherited from his mother, is said to darken and become more prominent when his thirst for truth and vengeance takes hold.