Aqua Hoshino, whose full first name is Aquamarine, is the central protagonist of the story. His existence is defined by a profound duality, as he is the reincarnation of a thirty-year-old medical doctor named Gorou Amamiya. In his previous life, Gorou was a dedicated physician working in a rural area and a devoted fan of the popular idol Ai Hoshino. After being killed by an obsessive fan on the night Ai was to give birth, Gorou was reborn as her son, Aqua, retaining all the memories and maturity of his adult life. This unique origin gives him an intellectual and emotional perspective far beyond his physical years, though it also becomes the source of his deepest psychological conflicts.

Physically, Aqua is a tall and handsome young man with blonde hair and distinctive aqua-blue eyes, which are the inspiration for his name. He inherited a signature star-shaped light in his right eye from his mother, a visual trait that often reflects his emotional state or deceptive intent. While he is generally seen as calm and composed, his personality underwent a drastic transformation following the tragic murder of Ai when he was just four years old. Before this event, he was a prodigious yet relatively normal child who enjoyed his life. Afterward, he became cold, calculated, cunning, and emotionally distant. He is driven by a singular, all-consuming goal: to find the man responsible for his mother's death—his biological father—and kill him. This quest for vengeance dictates nearly all of his actions, making him ruthless and pragmatic. He is willing to manipulate the emotions of those around him, viewing relationships and opportunities as tools to get closer to the truth, a trait that often makes his morality ambiguous. Despite this cold exterior, he possesses a deeply buried capacity for care, most visibly expressed through his overprotectiveness of his twin sister, Ruby.

In the story, Aqua navigates the entertainment industry as a means to an end. Though he worked as a child actor after impressing director Taishi Gotanda, he generally views his own acting talent as average. His true strength lies in his intelligence, quick thinking, and situational awareness, which allow him to elevate his performances and, more importantly, orchestrate his investigations. He has also worked behind the scenes, learning film editing and production from Gotanda. To uncover his father's identity, he strategically takes on acting roles, such as in the reality dating show LoveforReal and the stage play Tokyo Blade, using each opportunity to gather information, build connections, and conduct secret DNA tests.

His key relationships are complex and often strained by his hidden agenda. His bond with Ruby is paramount; knowing she is the reincarnation of his former patient Sarina Tendouji, he feels a profound sense of responsibility and guilt, leading him to be fiercely protective, sometimes to the point of being labeled a sis-con, or sister-complex. He frequently acts to support her idol career from the shadows while trying to keep her from his own dark path. His relationship with the talented actress Kana Arima is an old acquaintance from childhood; he respects her skill and cares for her genuinely, but he also manipulates her feelings and career path to serve his plans. With Akane Kurokawa, a fellow actor he saves from a career-ending scandal, the relationship is even more layered. He initially uses her for her incredible deductive abilities and her talent for mimicking Ai, entering a fake relationship with her. However, he develops genuine affection for her, leading to internal conflict and a desire to protect her from his dangerous mission, ultimately forcing him to sever ties to keep her safe. His mentor, Taishi Gotanda, is one of the few people who knows Aqua's past and supports his filmmaking ambitions, while his father, Hikaru Kamiki, remains the elusive target of his lifelong vendetta.

Aqua's character development is marked by his struggle between his desire for revenge and his yearning for a normal life. After a period where he believed his father was dead, he became noticeably warmer and began to entertain the possibility of a genuine future, particularly with Akane. However, the discovery that his father is still alive shatters this peace, plunging him into an even darker and more self-destructive state of mind. He becomes more calculating than ever, accepting that he may not survive his quest and believing that his actions are necessary to ensure Ruby's safety. His notable abilities include his high intelligence, which allows him to score exceptionally well on exams and solve complex problems like cracking phone passcodes. He is a skilled manipulator and planner, a capable actor, and a proficient video editor. He also retains his medical knowledge from his past life and is a gifted mimic, able to fool even his twin sister.