Sayaka is Akihiko's(name of player's character) co-worker & schoolmate in the Pia Carrot 3 game. They started working at #2 together & go to the same school. However, they haven't actually talked to each other before in school. It was only until they started working together that they noticed each other. You can say that she's the main leading female character in the game.
Sayaka has a secret crush on Akihiko & is too afraid to tell him. When she see him off at the train station for his transfer to #4, she felt heart broken as she lost the chance to confess her feelings to him. She's really a charming character. However, if you make the right choices in the game, Sayaka will actually come out to #4 and then Akihiko can then pursue a relationship with her.