Sawa Kannagi, twin sister of Towa, belonged to the family entrusted with safeguarding Mermaid Hill—a hidden repository holding a preserved mermaid corpse and derived substances. Designated heir, only Sawa knew its location after her father warned that mermaid flesh brought death or deformity, not immortality. When Towa fell terminally ill, Sawa retrieved mermaid blood, hoping to save her sister while secretly intending to test its effects for her own desire to retain youth and beauty. Towa consumed the blood, surviving her illness but suffering irreversible consequences: her right arm deformed into a claw-like state, and her hair turned white. Their father refused to acknowledge Towa’s condition, declared her dead, and confined her to a basement cell. Sawa complied, allowing Towa’s imprisonment while she married, had a child, lived a normal life, and eventually outlived her husband and son.

After their father’s death, Sawa released Towa. They lived together, though their relationship remained strained, with Towa harboring resentment and repeatedly demanding the location of Mermaid Hill, which Sawa refused to reveal. Decades later, when Dr. Shiina brought the immortal girl Mana to their home for Towa’s experimental arm transplant, Sawa witnessed Towa’s escalating cruelty, including attacks on Mana and Yuta. In a moment of conflicted guilt, Sawa freed Yuta and confessed her role in Towa’s suffering. Towa then kidnapped Mana, threatening decapitation unless Sawa disclosed Mermaid Hill’s location. Fearing for Mana’s life, Sawa relented and led them to the tomb.

At Mermaid Hill, Towa revealed her true motive: forcing Sawa to eat mermaid flesh, ensuring she endured either agonizing transformation or death, mirroring the suffering inflicted on Towa. Before this could occur, Sawa died suddenly of cardiac arrest, denying Towa her vengeance. In despair, Towa ordered the tomb and their home burned, with both sisters’ bodies consumed in the fire. Sawa’s actions stemmed from a blend of familial duty, selfish ambition, and eventual remorse, culminating in tragedy for both twins.