Kouko is Fuko's older sister. She used to be an art teacher at Tomoya's school, and she treats everyone kindly. Kouko is one of the familiar faces in the Furukawa Bakery, and Nagisa admires her very much, being her former art teacher. Currently she is engaged with Yusuke Yoshino.

Spoiler

and is the major reason why Fuko still exists in the school, despite still sleeping in the hospital.

In the movie, Kouko was a drama teacher instead of an art teacher. She, along with Youhei and Tomoya helped supervise and revive the drama club for Nagisa.