Mamiko is a recent college graduate along with her best friend Chieri. They have been teaching at Kouta's school since his second year. Mamiko is very friendly and popular among students. She serves as Chieri's guide through life and is a very reliable and dependable friend and mentor to Chieri.
Chieri confiscated Kouta's naughty magazine and brought it back to consult Mamiko. Mamiko pointed out that Kouta closely resembles Chieri's late younger brother and explained that Kouta may be upset that Chieri was using him to fill in the gap that was left. Mamiko also suspected that Kouta was interested in Chieri. Mamiko suggested that Chieri confront Kouta's parents about his slipping grades.
One day in Math class, Mamiko voiced her interest in Kouta and Shigeaki. She also noticed Kiyoka keeping an eye on her. Later if Kouta goes shopping with Kiyoka, Mamiko pointed them out and warned Chieri to not get too emotionally invested in Kouta.
When Chieri was absent from school due to a cold, Mamiko took over her class and gave the students homework. If Kouta approached Mamiko about Chieri, she gives him Chieri's home address.
Later when Kiyoka confronted Kouta about her relationship with Chieri, Kouta may request for Mamiko's help. Mamiko then confronted Kiyoka about it and supports Kouta's relationship with Chieri.