Chinatsu Iwasaki is a student at Toyo Private High School who participates in the summer school that serves as the central setting of Refrain Blue. Her background is tied to her involvement in the school's theater club, where she is a junior member.

In terms of personality, Chinatsu is described as an innocent girl who gets along well with others. She is friendly and has a tendency to follow people, suggesting a sociable and perhaps slightly dependent or eager-to-please nature. Her motivations often center around her apparent fondness for the male protagonist, as she tries to take care of him whenever something happens.

Her primary role in the story is that of a potential love interest, distinguished as being the first character that the protagonist can form a romantic connection with. One of her most significant relationships is with Harumi Sugino, a senior member of the theater club. The two have a close bond, getting along like real sisters. This connection is solidified early on, as they come to know each other because they were sitting in the same seat on the bus that brought them to the summer school location. The provided information does not include details on any character development arc or notable abilities she may possess.