Ritsuka's classmate. Despite her ditzy, bubbly exterior, she is sharp and goodhearted. On Ritsuka's first day in school, she immediately is overcome with awe and fascination. At the prodding of her mean-spirited classmates, she befriends him, although he is very reluctant. As Ritsuka overcomes his initial impressions of her and realizes she is rather smart, they become good friends.

Yuiko and Ritsuka's friendship greatly irritates their classmate, Yayoi. He had been recently rejected by Yuiko, although rather sorrowfully, claiming she only dated boys taller than her. Much to Yuiko's dismay, Ritsuka insists their friendship is platonic and it indeed remains that way. However, in two separate confessions to Shinonome Hitomi, Yuiko professes her love and devotion to Ritsuka. Their relationship is close, but Yuiko's seems to be a one-sided obsession. One incident of Ritsuka actually demonstrating care for Yuiko is in volume 2 of the manga and episode 4 of the anime, when she is bullied by her 'friends', whom Ritsuka fittingly said were using her. Because she had brought Ritsuka some homemade strawberry jam, they went so far as to smash the jar in front of her, and demanded she apologize for breaking school rules. Ritsuka came to her aid and stated that they were 'less than human'. This may have spurred on her love for him. Though, sadly for her, a notable scene in volume 4 depicts Ritsuka abandoning Yuiko abruptly to go to Soubi's aid, and Yuiko sadly recalls her efforts to become good friends with Ritsuka, but never achieving any results.

As the series progresses, a rather unusual yet harmonious friendship is struck between Yuiko and Soubi. Although it seems their personalities would clash, they have a unique understanding of each other.

In an early 2005 interview in Japanese magazine Pafu, Kouga Yun stated: "She emerged when I wondered, 'What kind of girl would it take for Ritsuka to be happy?' While she was created to match Ritsuka, she changes from meeting him...with some injection of intelligence (laugh). She honestly wants to change to become worthy of Ritsuka because she likes him. So if Soubi wasn't around, they'd make a great couple (laugh)."

(Source: Wikipedia)