Seiu is a member of the basketball team with Suruga Kanbaru. During her second year, she was the vice-captain. After Suruga retired due to the Rainy Devil's effects, Seiu became the captain.
Seiu is a student at Naoetsu High and is in the same year as Kanbaru. She is often blunt, calling Kanbaru an idiot for having bad grades. As a friend of Kanbaru and fellow teammate of the basketball team, Kanbaru and Seiu are presumed to be somewhat close friends. As Kanbaru's friend, she has given her the nickname "Ruga." Initially, Seiu seems to be a background character, though she has a more prominent role in Monster Season.
In Shinobumonogatari, her name is read as "Seiu," though in Yoimonogatari, it is later read as "Hoshiame," where both having similar meanings. This was later brought up in Amarimonogatari, and it is said that, "both readings are correct, you can read them both ways." There has been no further info about why Higasa's name is like this.