I don't blame you for not remembering since the chapter that focused on her was quite a while ago. (Chapter 6)
He had a childhood friend by that name and they were close, as they got older she came forward with the whole "Isn't it weird for a guy and girl to be friends" trope and cut him out of her life, the present her was confused as to why he keeps all girls at arms length and doesn't talk to her anymore but is fine talking to and interacting with Aoi.