From MangaHelpers:

Nanase Karina is an oddly cynical, sexually precocious 10-year-old girl, who has an unusual romantic relationship with her older, 12-year-old cousin, Tsukamoto Kouichi. Another classmate, Watanabe Kiichi, has a desperate crush on Karina, but he knows he can't compete with Kouichi, who has top grades, is the student council president, and much more. In their school life, Karina and Kouichi approach situations in a way that shows that they don't have the innocence commonly associated with children. For example, when Karina becomes the next targeted conquest of the school's playboy, she plans to beat him at his own game. [tethysdust]