Yoshiwara Sayaka is Mika's childhood tutor. She was hired to help Mika study for her entrance exams.
When Mika fell in love with her and tried to kiss her in her sleep, she returned the kiss and later asked her why she did it. She proceeds to make regular advances at a flustered but lovestruck Mika, until Mika catches her with another man. Yoshiwara defends herself by saying she's "just having fun", to which Mika responds that she no longer needs a tutor.
Later in the manga, she goes with her new girlfriend to Mika's school for their culture festival, and runs into Mika. They resolve their differences peacefully and go their separate ways.