Kouko is the older sister of Fuko and the fiancee of Yusuke. She is soft-spoken, polite, and patient in dealing with Fuko's eccentric behavior. Kouko is a regular customer of the Furukawa bakery and is a close friend of Sanae. She also used teach at Tomoya's school. Kouko is a friend of Koumura and Misae and she had taught Yusuke and Nagisa.
Kouko used to be the arts teacher at Tomoya's school. She retired 3 years prior and all the students during her teaching period had already graduated. Yusuke attended Kouko's art class. During Yusuke's second year of high school, he played a song on the guitar in class and she fell in love with him. Yusuke proposed to her and they got engaged. He then left to pursue his career in music. Some time later, Yusuke left the music industry and returned to Kouko.
2 years ago, Kouko sent her sister Fuko to her first year of high school. Fuko met with a road accident on her first day of school and she had been in coma every since. Fuko was admitted to a hospital in the neighboring town. Kouko and Yusuke visits Fuko frequently.
On April 26, Tomoya visited Kouko and congratulated her engagement to Yusuke and she was surprised that he knew because she told no one other than Fuko. Kouko also revealed that Fuko has always been at the hospital for 2 years. 3 days later, Kouko visited the Furukawa bakery. Tomoya wanted to confirm that Fuko was still at the hospital and she looked at him weird.
On Friday, Tomoya visited Kouko. She revealed that Nagisa enjoyed drawing Dangos and that she experienced difficulty in making friends. 2 days later, Tomoya visited Kouko again. She explained that Fuko is extremely shy and was constantly attached to her. She tends to run away when people try to call her. Later, Tomoya invites Kouko to the School Founder's Day.
That Sunday during the School Founder's Day, Kouko mentioned that she heard rumors that Fuko was at school. Kouko revealed that Fuko was still at the hospital and she was unable to perceive Fuko at the festival. Kouko requested Tomoya and Nagisa to be Fuko's friends in her dream. They requested Kouko to get married soon to fulfill Fuko's wish.
Several days later, Kouko and Yusuke held their wedding at school. Fuko then appeared before Kouko and gave her a wood starfish.
Kouko visited the Furukawa bakery before she visits Fuko. She sent Tomoya to visit Koumura.
After Tomoya and Nagisa had heard Yusuke's story, Nagisa borrowed one of Yusuke's album CDs and approached Kouko. Kouko revealed that she wanted Yusuke to continue to pursue a career in music and suggested that he find a career in the indie music scene.
During the holidays, Tomoya and Ushio visited the Furukawa bakery and found Sanae, Kouko, and Yusuke in the park. Fuko had recently been discharged from hospital and Kouko introduced Fuko to Tomoya and Ushio. Kouko and Yusuke then held their marriage at school.