Kozue is the daughter of a wealthy family. She is a close friend of Misaki and idolizes her. Kozue carries a digital camera everywhere which she uses to capture every special moment of Misaki. She maintains a large album that chronicles Misaki's life. Kozue also commands a formidable intelligence network.
Most people call Kozue by her last name. Kozue constantly recommends Misaki as a girlfriend for Yuuji. She is also very protective of Misaki. Kozue likes Assam tea.
Rei and Kozue deduced that Sumiyoshi has feelings for Misato.
For the culture festival, Kozue provided the filming equipment to film the movie and edited the movie.
On Christmas Eve, Kozue followed Yuuji out of his home. She revealed that Misaki has had feelings for Yuuji since he transferred to Sakuragaoka. Yuuji confessed his feelings for Kozue. Kozue was surprised and does not accept it.
On New Year's Day, Yuuji invites Kozue to Yukito Shrine. He confessed his feelings again under the Snow Sakura and Kozue accepted it reluctantly. Later, Kozue was conflicted between her feelings for Yuuji and her desire to pair Misaki up with him. She then wanted Yuuji to keep their relationship secret from Misaki.
On Kozue's path, she revealed that she started an album on Yuuji.
On Valentine's Day, Misaki gave Yuuji some chocolate. Kozue felt guilty about her relationship with Yuuji and Yuuji calmed her down. She then invited him to her home to watch the album that she made about him.
That Sunday, Yuuji called Kozue for a date. Later, Yuuji caught the flu. Kozue visited him to take care of him. Later still, Saki suspected Yuuji's relationship with Kozue and she accidentally lets slip of the fact to Misaki. Misaki confronted Kozue on the school's roof. Misaki exploded in rage and Kozue buckled under her wrath.
Rei figured out how to fix Misaki and Kozue's friendship. She employed Misaki's sister Misato to talk to Misaki. Saki and Sumiyoshi volunteered to talk to Kozue. Misaki managed to reconcile with Kozue.
Later, Kozue revealed that she will be furthering her studies overseas. Misaki planned a date for Kozue and Yuuji before she left.
4 years later, Kozue returned to meet Yuuji under the blooming Snow Sakura.