She is the main character, and the story is mostly told from her point of view. She also has the Medusa virus. Her twin sister Shizuku, who also had the disease, was not chosen to go into cold sleep.

Part of the manga focuses on Kasumi coming to terms with what she feels was abandoning Shizuku, even though Shizuku was the one who encouraged her to go. In the short glimpses the reader is given of Kasumi's life prior to awakening in the treatment center, it appears that she suffered from self-esteem issues, and was frustrated with herself for not being able to live up to her sister's self-confidence and popularity.

Kasumi also has a crush on Marco, which is visible enough that Katherine feels the need to warn Kasumi that although it is understandable that Kasumi would like Marco, she "mustn't fall for anyone" if they can't trust her back.