Kosaki is a kind, innocent and sweet classmate of Raku's and Ruri's best friend. Although Raku has a major crush on her, she does not reveal that she has a similar crush on him. When Raku mentions his lost locket of his first love, she shows some interest as she secretly carries a potential key, however, when confronted about the key or being the first love, she lies and denies it in embarrassment. Although she works with her mother at a dessert shop, she is a terrible cook on her own. Surprisingly, whatever food she makes looks good on the outside.