Yuiko Hawatari (羽渡唯子 Hawatari Yuiko) is Ritsuka Aoyagi's classmate. At 167 cm tall, Yuiko is much taller than Ritsuka and her other classmates and because of this and her unusually large breasts, she is often mistaken for being older than she actually is. Despite her ditsy and bubbly exterior, she is sharp and goodhearted, though Ritsuka initially thought her annoying. Ritsuka also stated that he doesn't like people that are being used by others, which happens to Yuiko in the first episode.
On Ritsuka’s first day of school, Yuiko is immediately overcome with awe and is fascinated by him. At the prodding of her mean spirited classmates, she befriends him, though he is very reluctant to accept her because he thinks she is weird and unintelligent. As Ritsuka overcomes his initial impressions and realizes she’s quite smart, they become good friends much to the irritation of another classmate, Yayoi. Yuiko had recently rejected him, claiming she would only date boys taller than her, and he is jealous of the effect Ritsuka has on Yuiko.
Much to Yuiko’s dismay, however, Ritsuka insists their relationship is platonic and it, indeed, remains that way. Still, Yuiko confesses her one-sided love and devotion to Ritsuka on two separate occasions to their teacher, Hitomi Shinonome.
Although it is one sided, Ritsuka still cares for Yuiko, standing up for her when in volume two of the manga and in episode 4 of the anime, Yuiko is bullied by her ‘friends’ after bringing some homemade strawberry jam to school for Ritsuka, which is then smashed in front of her by the classmates who Ritsuka fittingly says are using her. However, a notable scene in volume 4 shows Ritsuka abandoning Yuiko adruptly to go to Soubi’s aid, and Yuiko sadly recalls her efforts to be good friends without achieving any results.
Yuiko has a fear of heights, indicated in episode 11 of the anime whilst on a ferris wheel with Ritsuka. As the series progresses, an unusual yet harmonious friendship is struck between Yuiko and Soubi, and although it seems their personalities would clash, they have a unique understanding of each other.