Matsui is Haruko Akagi's friend who, along with their friend Fujii, usually accompany Haruko to Shohoku's basketball matches and practices. Unlike the shy Fujii, Matsui has more of a blunt personality. She's usually the more logical and candid one out of the trio.
Hanamichi Sakuragi calls her Haruko's friend #2, but she quickly corrects him with her actual name. As soon as Haruko lifts Sakuragi's spirits back up after Shohoku's loss to Ryonan, Matsui remarks how much of a simpleton Sakuragi is.