Goro Amemiya is the original identity of Aquamarine Hoshino, one of the central characters. Before his reincarnation, Goro was an obstetrician-gynecologist in his mid-thirties working at a hospital in the rural town of Takachiho in Miyazaki Prefecture. His life before becoming a doctor was marked by tragedy, as his mother died from blood loss shortly after giving birth to him at home, having hidden her pregnancy from her own parents. With no knowledge of his father's identity, Goro was raised by his grandparents, though his relationship with his grandfather was cold due to the circumstances of his birth. Inspired by a novel featuring a heroic doctor, he initially aspired to become a surgeon, but he ultimately chose to specialize in obstetrics and gynecology, partly influenced by his grandmother's wishes.

Possessing a dedicated and compassionate nature, Goro was deeply committed to the well-being of his patients. This devotion is most clearly seen in his care for a young patient named Sarina Tendouji, a girl suffering from a terminal brain tumor. While she was under his care, Sarina, a fervent fan of the idol Ai Hoshino, shared her dream of being reincarnated with a face like Ai's. Goro grew close to her, promising to consider her proposal of marriage when she turned sixteen and to take her to Shibuya once she recovered. When Sarina’s condition became critical and her neglectful parents failed to appear, Goro stayed by her side. In her final moments, Sarina gave him an Ai Hoshino badge, and her death left a profound impact on him. Seeing the shadow of Sarina’s dreams in the idol Ai, Goro became a devoted fan himself, wanting to see Ai succeed on the path Sarina had dreamed of walking.

Years later, Goro's role as a fan and a doctor collided when the very idol he supported, Ai Hoshino, arrived at his rural hospital, twenty weeks pregnant and seeking discretion. Despite initial internal conflict over the situation, he was moved by Ai’s determination to continue her idol career while also embracing motherhood. Goro chose to support her as both her physician and a fan, promising to safely deliver her twins. This role placed him in the crosshairs of a dangerous stalker who had followed Ai to the hospital. On the night Ai was due to give birth, the stalker confronted Goro and pushed him off a cliff, resulting in his death. On that same night, Goro was reincarnated as Ai’s son, Aquamarine Hoshino. His body remained undiscovered for a long time and was later found in a skeletal state.

The key relationships of his former life continued to shape his new one. His bond with Sarina was foundational; her wish to be reborn as an idol’s child was fulfilled when she was reincarnated as Aqua’s twin sister, Ruby Hoshino, though neither initially recognized the other. His connection to Ai Hoshino became a dual source of admiration and tragedy, as her murder when Aqua was four years old gave him a new, dark purpose in his second life. As part of his development, the guilt and trauma Goro carried often manifested as a separate persona within Aqua’s mind, tormenting him over his perceived failure to protect Ai. As a doctor, Goro possessed notable medical abilities, specifically in obstetrics, which he demonstrated through his professional handling of a high-profile pregnancy. He also showed courage and a protective instinct, willing to confront danger to safeguard others. His unresolved feelings about his own birth and the loss of his mother and Sarina were central drivers of his personality and later, his actions as Aqua.