Yagi was a top player at a school with a high-ranked tennis club, and currently goes to the same college as Kento Sawada.

Keito and Yagi used to date. They started to date right after Keito arrived at college, and they started to train together. He even brought Yagi to one of the Tennis club's practice after school. They broke up due to her cheating on him three times before he noticed. Afterwards he started to date Shou Iketani.

Although Keito and Yagi broke up, they eventually started to see each other once again. This time, while Keito was dating Shou, he was the one who was cheating. He kept making up excuses to see her without wanting Shou to find out. Eventually she did find out. Even once, when Seiji Utsumi and Yuki Kurokawa went to a concert, there Seiji saw both of them. Seiji confronted Shou about her unhappy appearance, and she went to Keito's apartment. There, she found Yagi's pantyhose in thee trashcan. The day after, Seiji and Shou were walking together and saw Keito closing the car door for Yaji. After this incident, Shou decided to break it off with Keito due to his affair.