A nice but dorky classmate of Ritsuka's. Yayoi, Ritsuka, and Yuiko have a sort-of, would-be love triangle thing going on, but Yuiko's apathy towards Yayoi and Ritsuka's apathy towards Yuiko and Yayoi make the triangle simply a possibility. Even though he was rejected, Yayoi doesn't give up on Yuiko.
Like Katsuko, Yayoi is only present for a few scenes here and there, and is not very fleshed out. In the anime adaptation, Yayoi is around much more, and acts pretty over-the-top and goofy.