Hikaru Kamiki is a central figure in the narrative who serves as the primary antagonist. He is the biological father of the twins Aquamarine and Ruby Hoshino, and the former boyfriend of the idol Ai Hoshino. His actions, directly and indirectly, are the catalyst for the story's central tragedy and the driving force behind his son Aqua's quest for revenge. Possessing a handsome and captivating appearance, he bears a striking resemblance to his son Aqua, with long blonde hair and a tall stature of 178 centimeters. Like Ai and his children, Hikaru has star-shaped pupils in his eyes, which are most often depicted as being black in color, reflecting his dark nature.

Hikaru's background is marked by severe childhood trauma that fundamentally warped his psyche. During his early teens, while a member of the theatrical troupe Lalalie, he was repeatedly sexually abused by an older actress named Airi Himekawa. This abuse began when he was around ten or eleven years old and resulted in Airi giving birth to his son, Taiki Himekawa. This prolonged victimization led to deep-seated issues with attachment, trust, and self-worth. He developed a disdain for his own good looks, believing them to be the reason he was preyed upon.

His personality is a complex and deceptive duality. On the surface, he can appear as a polite, meek, calm, and dependable individual, a facade he maintains skillfully thanks to his background as an aspiring actor. However, this masks a deeply disturbed and callous interior. He is a sadistic individual who derives a sense of life's weight or pleasure from the act of killing, particularly targeting talented and valuable actresses. His trauma left him emotionally stunted, and he never truly matured past the point of his initial abuse, remaining an immature and twisted person obsessed with a delusional concept of love. Hikaru is also a master manipulator, preferring to operate from the shadows. He uses proxies and psychological manipulation to commit his crimes, ensuring he does not get his hands dirty and leaves no incriminating evidence. He is highly cautious, attacking victims in isolated locations and making deaths appear as accidents.

Hikaru's motivations are rooted in his broken perception of love and Ai Hoshino. He became obsessed with Ai, feeling that he could only truly feel alive when he was with her. After their relationship ended, he became trapped in a delusion that killing others who might surpass Ai in talent or brilliance would add weight to the love he felt for her. He spreads this distorted ideology to others he manipulates. His actions are driven by a desperate, psychotic need to feel connected to Ai and to validate his own existence through death. He is incapable of healthy love or attachment, viewing relationships as a source of obsession and destruction.

Hikaru meets Ai Hoshino while both are members of the Lalalie theater company. They begin dating, and when he is just fifteen years old, Ai becomes pregnant with the twins. Seeking comfort, he confesses his childhood trauma to Ai, but she rejects him, calling him weak, which deepens his despair. After Ai leaves him, he is implied to be the one who leaked her pregnancy and her new address to a obsessed fan named Ryosuke. This directly leads to the stalker's attack and Ai's murder. He maintains a distant and sinister presence over his children. He visits Ai’s grave years later and crosses paths with Ruby, commenting that she is beautiful because she is their daughter. He eventually moves to harm Ruby, seeing her as someone who might shine even brighter than Ai. His ultimate confrontation is with his son Aqua, who has dedicated his life to tracking him down for revenge. Hikaru also fathered a son, Taiki Himekawa, with his abuser Airi, a fact Taiki is initially unaware of. The actress Akane Kurokawa is instrumental in deducing Hikaru’s identity as the mastermind behind Ai’s death, using her keen analytical skills. He also manipulates Niino, a former colleague of Ai’s, and her boyfriend Ryosuke to carry out his original plan.

Throughout the story, Hikaru's character development is less about change and more about the gradual unveiling of his true, static nature. Initially a shadowy, mysterious figure, his past as a victim of abuse is revealed, providing context but not excusing his actions. He is shown to be a serial killer, as he later murders the popular actress Yura Katayose, pushing her from a mountain path. His appearance in the final arc reveals that despite the passage of nearly fifteen years since Ai's death, he has not grown or changed at all. He remains a person who cannot reflect on his mistakes and only feels obsession and regret for Ai when confronted with his own death. He is the antithesis of Ai: while she lied to give love and make others happy, Hikaru lies and manipulates solely for his own selfish and destructive ends.

Hikaru's abilities are not physical but psychological. He possesses a genius-level intellect for manipulation and a charismatic presence that allows him to easily influence and control vulnerable people, turning them into unwitting pawns. He is a pathological liar, skilled at maintaining a charming public persona while orchestrating tragedies from behind the scenes. His training as an actor allows him to flawlessly mask his true murderous intentions. When physically confronted by Aqua, he is shown to be helpless and panicked, relying on his wits and manipulation rather than any physical prowess. His true weapon is his ability to corrupt and manipulate others to do his bidding without ever facing consequences himself.