I mean she may not even have a remaining role. Summer break is over it seems, and they only really reconnect during summer break. Unless a year in universe goes by without anything happening she probably won't have a major role to play. Also I don't know why so many comments assuming she is going to be some kind of love rival. This just doesn't seem like that type of manga.But they cant just ignore the character. Whats her remaining role gonna be
Thats why I think she probably just served to contrast this chapter. Instead of trying to wall himself off from her like he previously did with past friends. He is accepting of their current relationship, and wants to remain friends. Showing that he has improved in his mentality. Unless she somehow transfers from her private school I doubt she has a major role to play.