I'm Yoimiya!

Ayaka has grown up with Ayato (her brother) being her only friend. But, as soon as they get to highschool, Ayato and her suddenly become popular because of their status as Kamisatos. Ayaka isn't so fond of this attention... But Ayato doesn't seem to mind. Ayato ends up getting the reputation of 'calm', 'cool', and 'sophisticated', while Ayaka begins to feel like she's lost her only friend. But one day, an energetic blonde girl comes up to talk to her, seemingly out of nowhere.
"Hiya! My name's Yoimiya!"
PS: The title is also the name of an Ayamiya fan song! Feel free to look it up if you're curious lol