Yuika Koga (古賀 唯花, Koga Yuika) is one of the main characters of the series. She is in the first year of High School and a potentiel Cinderella.
Yuika is 1/4 English and therefore has blonde neck-length hair and green eyes. She is short in stature and quite petite with a flat chest. She has been described by Keiki as the cutest thing he has ever seen, she is also quite pretty and attractive. She is usually seen wearing her uniform and a green ribbon on her head, but she has worn other cloths when on a date with Keiki.
On the outside she is a cute and outgoing girl who is playful with her friends, especially Keiki. She can sometimes be childish and jealous when Keiki is around other girls, she especially does not like it when it is Sayuki. She does not get along with Sayuki and is always picking on her and the same goes both ways.
However on the inside Yuika is actually a huge Sadist/Femdom. She wants to dominate Keiki and wishes for him to become her slave and she his master, the polar opposite to Sayuki.
Yuika enjoys Keiki's company and cares for him. He helps out in the library alongside her and they playfully chat when together. He is quite protective over her as shown when she was cornered by some players trying to pick her up, Kiryuu protected her by taking her hand and running off.
Yuika first met him in the library whilst reading by her lonesome. At first she was cold and uninterested in Keiki, but he persisted to talk to her when she was alone over and over again, and each time she began to warm up to him which eventually led to her becoming more comfortable around him and the pain of loneliness was forgotten because of him. It is suggested that she grew feelings towards him after this and is shown to becomes jealous when her he is friendly with other girls especially Sayuki.
It is revealed that Yuika's 'romantic feelings' for Keiki are just as 'twisted' as Sayuki's. In contrast to Sayuki's Masochistic side, Yuika is a complete Femdom/Sadist deep down and confessed this to Keiki after their date. She wants to Dominate him and become his master. She wants him as a slave/servant.
Sayuki and Yuika are always at odds against one another and appear to have a rivalry for an unknown reason. They are polar opposites in appearance and personality. They enjoy picking on one another and compete for Kiryuu to an extent as they are shown to get jealous over the other if Keiki does anything with them, especially Sayuki. They are at odds due to them both claiming ownership over Keiki when they first met Yuika as his master and Sayuki as his pet.