Ishiki, Kasumi

カスミ・イシキ

She serves as the protagonist, with the narrative primarily unfolding from her perspective. She is also infected with the Medusa virus. Her twin sister, Shizuku, who suffered from the same illness, was not selected for cryogenic suspension.
A significant portion of the manga deals with Kasumi grappling with the guilt of what she perceives as abandoning Shizuku, despite Shizuku having urged her to proceed. In the brief flashbacks depicting Kasumi's life before her awakening at the treatment center, she appears to have struggled with low self-esteem, feeling frustrated by her inability to match her sister's confidence and popularity.
Kasumi also harbors a crush on Marco, a sentiment evident enough that Katherine feels compelled to caution her. While acknowledging that Kasumi's attraction to Marco is understandable, Katherine warns that she "mustn't fall for anyone" if she cannot be sure of their reciprocal trust.