(from ebookjapan):
Katsuko Kazahaya, now a high school student, lost her mother and came to Tokyo to live with her aunt. She meets her cousin Kyo, who is the same age as Katsuko, at the Sakura family. Kyo is so lively that she mistakes her for a boy. Katsuko and Kyo go to the same high school. Katsuko is intrigued by Kyo, who is very manly. One day, Katsuko realizes that the bruise on Kyo's neck is a secret between father and daughter...?