Nomiya Nanako

A neighbourhood girl the protagonist tutored when he himself was still a student.
She developed a crush on him from back then, even now she and the protagonist keep in good touch.
Because she's known the protagonist since childhood she has unwavering confidence that she'll never lose him to anyone.
Other than the strong feelings she has she's an utterly ordinary girl.

Even her Academic and Athletic grades are unremarkably average. Yet, the protagonist is interested to see how much she's matured since childhood into a "woman".