Shiina, a physician affiliated with the Kannagi family, first encounters Towa Kannagi as a young doctor entangled in a romantic bond that solidifies into a lifelong partnership defined by loyalty and unresolved longing. Over decades, he devotes his medical skills to mitigating Towa’s agony from a mermaid-blood-induced arm deformity, repeatedly grafting replacement limbs harvested from deceased girls onto her body. These temporary solutions decay cyclically, mirroring Shiina’s own aging against Towa’s unchanging form. His access to corpses—obtained through morally dubious means, such as covertly procuring Mana’s remains after her accidental death—facilitates these macabre procedures.

Guilt, perceived obligation, and an enduring emotional tether compel Shiina to sustain Towa’s existence despite witnessing her psychological unraveling. He observes her manipulation of Sawa and coercive efforts to impose immortality on others, yet persists in enabling her descent, conflicted but unwilling to abandon her. During the mermaid’s tomb confrontation, he confesses his futile yearning to flee with Towa, exposing his awareness of their inescapable tragedy.

Following Sawa’s demise, Shiina collaborates in demolishing the Kannagi estate, opting to share Towa’s self-immolation. Their fiery deaths encapsulate his compulsive devotion, framing him as a figure shackled by love and complicity in the fallout of mermaid experimentation. His narrative arc remains inextricably tied to Towa, chronicling the erosion of ethics under the weight of unwavering allegiance.