Kasumi Kurose is a female student standing 158 cm tall with ochre-colored hair and eyes. Her birthday is July 14, placing her under the Cancer zodiac sign. She wears a standard school uniform and sports twin ahoge (hair antennae) as a distinctive hairstyle.

Childhood friends with Moyako Konoe, they attend the same class. Kasumi frequently hears Moyako's complaints about Moyako's younger brother, Akira, whom Moyako believes has a sister complex. While Kasumi typically dismisses these concerns as Moyako's exaggerated imaginings, Moyako remains convinced of Akira's supposed perversion. Kasumi maintains a friendly and understanding relationship with Akira, which further fuels Moyako's belief that Akira manipulates Kasumi into compliance.

Kasumi serves as Moyako's reluctant confidante, often involved in Moyako's delusional interpretations of mundane interactions. During school lunch breaks, she witnesses Moyako spiraling into paranoid monologues about Akira's innocent actions, like sharing identical lunch bento boxes. Kasumi correctly identifies a mark on Akira's face as resulting from sleeping in class, countering Moyako's assumption of inappropriate origins.

Her role involves mediating or inadvertently escalating situations stemming from Moyako's overactive imagination. When Moyako panics during swimsuit shopping over perceived peeping risks from Akira, Kasumi receives a cryptic distress text. Misunderstanding the context, she shows the message to Akira, inadvertently causing a physical mishap in the changing room. Kasumi also visits the maid café where Moyako works, prompting Moyako to misinterpret Akira's presence as part of a scheme to exploit her subservient role. Documented media focuses on these interactions.