Hiroki wields psychic abilities focused on memory manipulation, visually expressed through a persistent goldfish motif. His powers enable him to enter minds, navigate memories categorized as cherished "Peaks" and traumatic "Valleys," and alter perceptions. During these mental journeys, he physically transforms into a goldfish, retaining only his human eyes while developing fin-like hair and serrated teeth.

Confined within a neglected psychiatric ward during childhood, his earliest recollections involve staring into a goldfish bowl before terrifying psychic episodes overwhelmed him, filled with dark voids and red electrical currents. During one such episode, Tsukasa entered his consciousness, forming Hiroki’s foundational "Peak" memory and granting emotional stability. This event solidified Hiroki’s desire to become a goldfish and forged his deep bond with Tsukasa. Later, within Tsukasa’s mental "Locus," Tsukasa taught him the necessity of using "Valleys" to protect one’s "Peak."

Emotionally expressive and perceptive, Hiroki exhibits intense mood shifts and childlike wonder, ranging from joyfully spraying water to tearful outbursts. He openly resents entering others’ "Valleys" due to his strong empathy for their pain. Though he often requires explanations for practical matters, like separating fighting fish, he demonstrates sharp insight, suspecting organizational conspiracies and improvising under pressure. His naivety surfaces in unrealistic expectations, such as believing he and Tsukasa could easily leave their criminal organization.

His relationship with Tsukasa defines his motivations. Hiroki views Tsukasa as his "Peak" giver and craves a peaceful life together, symbolized by their shared dream of running a fish shop, reflecting his deeper desire for mutual independence. However, Tsukasa treats him as a subordinate "pet," creating a power imbalance. Hiroki exhibits jealousy when Tsukasa interacts with others and prioritizes Tsukasa’s presence over business, becoming despondent when left alone at the shop.

Initially compliant with their work for "the Company," Hiroki grew disillusioned after a mission involving Kenji. He opposed murdering targets, advocating instead for memory alteration, successfully persuading Tsukasa and their handler Katsuragi. Witnessing the moral cost of manipulating Kenji’s cherished bond intensified his resolve to leave. When Tsukasa continued accepting dangerous assignments, Hiroki managed their fish shop alone but neglected it, fixated on Tsukasa’s absence.

Discovering Tsukasa’s role in crushing Hayashi fractured their relationship. Hiroki confronted Tsukasa’s violent tendencies and suggested escaping together, but fled after witnessing Tsukasa’s volatile anger. While hiding, he reflected that Tsukasa appeared "trapped" and returned, offering to replace Hayashi as Tsukasa’s "Peak" giver through memory alteration. He protected Tsukasa by withholding his culpability from Satoru and submitted to captivity when accused, insisting he wouldn’t abandon Tsukasa.

In the climax, cornered by Satoru seeking revenge for Hayashi’s crushing, Hiroki rejected Tsukasa’s framing of the event as part of a plan to crush Satoru. He severed his goldfish image to escape. Physically captured and restrained, he survived the organization’s collapse alongside Satoru. They resolved to rescue Tsukasa, Hayashi, and others, continuing Hayashi’s mission to dismantle the Company.

Hiroki’s appearance features medium-length, orange-tinged hair, lighter at the crown, and caramel-colored eyes. He typically wears a red-and-white T-shirt with geometric patterns, layered shorts, red sneakers with blue accents, and a unique pendant. These elements remain consistent across official media.