Kasai has short Persian orange hair with similar-colored eyes and she wears a light brown cardigan like a school uniform with a white buttoned undershirt and a red string tied in a bow around the collar.
Kasai appears to be a generally friendly person but can be petty, jealous, and spiteful although not to the extent that is prevalent. It is suspected that she may have had feelings for Ritsuka and was annoyed at how it was left unreciprocated as Ritsuka does not seem to acknowledge her other than a classmate. Kasai is also quite a jealous person, as shown when she shows frustration at Ritsuka suddenly getting close to Mafuyu and at Mafuyu almost always stealing his attention. But, she is apologetic for her actions, especially after telling Ritsuka about rumours she heard about Mafuyu during middle school. After the concert, she seemed to have mellowed out her feelings towards Ritsuka and Mafuyu's relationship, as she had encouraged Ritsuka to visit Mafuyu when he got sick.
Kasai and her friend, Waka, walk together through Shibuya only to spot Ritsuka and Mafuyu walking on the other footbridge. Upon seeing Ritsuka pull Mafuyu close to keep the latter from bumping into other pedestrians, Kasai expresses her frustration at how they're always together and questions if they were dating or not. Waka happens to know something about Mafuyu and Kasai begs to tell her.
- Her full name is first revealed in the ending credits of the anime's second episode.