Aiko is a girl one year older than Shinichirou who attends a different school than him. She is going out with his best friend Miyokichi Nobuse.

Spoiler

But she may have feelings instead for Shinichirou hinted through a comment she made about if people can't become closer to the one they like, they'd like to get closer to the one closest to that someone . Aiko has known Shinichirou and Miyokichi since they were kids. She helps out at her family's Imagawayaki shop named Imagawayaki Ai-chan. She is shorter than Shinichirou but enjoys teasing him from time to time.

(Source: Wikipedia)