Shizuku Ishiki
is Kasumi's twin sister, identical to her if not only for their hairstyle, the fact that she didn't wear glasses, and her upbeat and cheerful personality. Infected with Medusa, despite being left out of those chosen to be cured, among who there was her own sister, she still greatly supported her and insisted for her to take the chance to be saved from Medusa.

However, their bond grew increasingly strained after Shizuku witnessed Kasumi trying to kill herself, and again when, before parting with each other, Kasumi wished for them to commit suicide together, being incapable to accept a world without her twin. During the ensuing struggle Shizuku accidentally pushed Kasumi over the cliff they were standing, and shocked at the death of her sister, awakened her Medusa power.


Monster Shizuku

Transported into Level 4, still overwhelmed by the memory of her sister's death, Shizuku created a countless amount of abominations, initially blobs of quivering flesh, maws and claws which laid waste to the labs while massacring the staff, and only through Zeus' mind-controlling program conceiving the creatures that would populate the ruined island, such as the dinosaurs encountered by the protagonists at the beginning. Also, it is revealed that the monstrous behemoth encountered at the beginning of the manga, and later shown occasionally as the "mother" of the creatures, was Shizuku herself, the beast being an unconscious armor to seal herself from the world.

While often mentioned by Kasumi during the course of the story, Shizuku appeared often, other than in her monstrous form, through ominous apparitions, often Kasumi's delusions or unconscious mental links between the sisters, and only in the end she appeared at the core of her mutated form, begging Kasumi not to uncover the fact that she was only a Medusa-made creation, and that her true self had died. With the death of Zeus, she too ceased to exist, her monstrous body crumbling to pieces and her essence fading away peacefully.