Dr. Shiina was the fiancé of Towa Kannagi and devoted his life to her care. Towa suffered physical and emotional torment after consuming mermaid blood, an act orchestrated by her twin sister Sawa. Though it cured her terminal illness, it deformed her right arm monstrously and turned her hair white prematurely, forcing decades of isolation beneath the Kannagi estate.

As her physician, Shiina treated Towa during her confinement. Her deformed arm caused chronic, severe pain, leading Shiina to repeatedly amputate the right arms of recently deceased young women—sourced from his clinic or graves—and transplant them onto Towa. Each grafted arm degenerated over several years, necessitating the gruesome cycle.

His actions escalated when Mana, an immortal mistakenly declared dead after a truck accident, was brought to his clinic. Intending to use her arm for Towa, he was shocked when Mana revived. Later, Towa demanded he decapitate the living Mana to facilitate a head transplant, but Shiina refused to operate on a conscious person.

He accompanied Towa and Sawa to Mermaid Hill, where Towa sought to force Sawa to consume mermaid flesh as revenge. After Sawa died of cardiac arrest, thwarting the vengeance, Shiina fulfilled Towa's final request to burn the tomb, the Kannagi house, and all related artifacts. Amidst the conflagration, he reflected on his decades of unreciprocated devotion, realizing her hatred for Sawa had consumed all thought of him or his sacrifices.

In the OVA adaptation, he pursued Towa into the burning Mermaid Hill, choosing to die alongside her to make up for "fifty-five years of lost time."