Kasai is a character in the Given series, and a classmate of Ritsuka.
Kasai has short persian orange hair with similar colored eyes and she wears a light brown school uniform with a white undershirt and a red string tied in a bow around the collar.
She is very honest and truthful about her feelings towards others, but can become jealous over other people's relationships, especially the friendship between Ritsuka and Mafuyu. She discussed matters openly with her friends and was the first to have some notion of Rituka's feelings concerning Mafuyu. Kasai became tearful feeling that she had said something unnecessary to Rituka.
Inviting herself into the conversation Shogo is having with Ritsuka, Kasai has observations of her own to add to Shogo's questioning of Ritsuka on the time he is spending with Mafuyu. Pointing out that neither Ritsuka or Mafuyu are sociable at all, she asks if she is right before continuing that they are in different classes, have different styles and asks what is up. That is the hot topic among the girls right now and Kasai continues that Mafuyu is cool and has a lot of secret fans but seems hard to approach, but now has suddenly gotten all attached to Ritsuka, just as Mafuyu himself appears at the class door.
Yasai calls Ritsuka and Shogo out when the former attacks the latter with a broom over his suggestion he was not expecting Mafuyu to turn him down. She pays close attention to the way Ritsuka has reacted to this rejection as well.
In class when Shogo and Ryou want Ritsuka to awaken to get lunch, she tells them she will go and wake him up. Prodding him, she jumps in surprise at Ritsuka suddenly rising from his desk only when Mafuyu calls him. Her friend points out she is making quite the face when Ritsuka immediately moves to speak to Mafuyu instead. Not understanding it she hears her friend went to the same middle school as Mafuyu where there were rumors that he was in a relationships with another guy.
She is remembered trying to wake Ritsuka when Mafuyu wants to but cannot find the words to raise with Ritsuka.
Reiterated what she said concerning Mafuyu and Yuki Kasai then lowers her head and says sorry. She does not react to Shogo sharing Ueki's drawing, something he finds hysterical, instead woefully shares her feeling that the last thing said was unnecessary. It was just a rumor they were dating but regarding Mafuyu’s friend dying that is true drawing upon her friends background of attending the same secondary school with both boys and reveals the name Yuki.
During clean up Kasai is not overly fond of being tasked by someone else with taking a garbage bag simply because the disposal area is one her way as she changes rooms for next period. She is surprised to see that Ritsuka is here to take the chore out of her hands as he offers to take the bag for her on account it would be heavy carrying it all by herself.
Following him with her head lowered Kasai only confirms where the disposal area is when Ritsuka asks. Outside with the deed done Kasai stands teary eyed and asks Ritsuka why he is being so kind to her after she told him such a cruel thing. Kasai reminds him it was regarding Mafuyu when Ritsuka is clueless as to what she is referring to. Kasai kept thinking it was so sly for her to tell him in that manner and as she sobs she thought he would never talk to her ever again.
Looking up in dread as Ritsuka begins to voice that it might have been sly, Kasai hears he was glad to find out about that and she has her hair patted affectionately by him. She is unaware her exchange has been innocuously viewed by Mafuyu himself along with Shogo and Ueki.
Kasai and her friend walk together through Shibuya only to spot Ritsuka and Mafuyu. Her friend happens to know something about Mafuyu and begs to tell her.
From Japanese 火災 meaning fire or flame.