Rio Kojima is a minor character in the Sekai-ichi Hatsukoi series. She is a Touto University student majoring in fashion, and a close friend of Kou Yukina.
After missing a date, Shouta Kisa intends to catch Yukina after work, but Yukina meets up with Rio. Unnoticed, Kisa follows them far enough to see them walk into the hotel district, which is known to be mainly for lovers. This leads Kisa to believe that Yukina is seeing her. Yukina later explains that Rio is a friend from University, and in exchange for teaching him how to cook, he took her to a movie. The theater happened to be in that area.
She is later seen in front of Yukina's front door ringing on the door bell.
Kojima and Yukina are good friends. She is unaware of Yukina dating Kisa and doesn't know that many of Yukina and Kisa's fights are sometimes about her. Rio taught Yukina how to cook decent meals in exchange for him going to slasher marathons with her. She and Yukina had dated before, but their relationship ended in a mutual breakup since they preferred being friends.