17 year old Kippei is a popular high school Casanova, who skips out on class to flirt with girls. He's not serious about his relationships with his fan girls, not serious about school -- not particularly serious about anything really. However, Kippei's carefree days come to a screeching halt. His aunt disappears suddenly, and his 5-year old cousin Yuzuyu comes to live with his family. Kippei's bossy older sister, annoyed with Kippei's free and loose behavior, saddles him with the responsibility of taking care of Yuzuyu.
It's a big change for Kippei. Between taking her to kindergarten, picking her up, and watching after her, he has no time for the big girls he used to play around with. At first, he just feels sorry for her, but it turns into a genuine affection, and he comes to enjoy doing things for her. He evens learns how to prepare a proper lunch because of her (when the story begins Kippei doesn't even know how to make a rice ball- how stupid is that?)! As for Yuzuyu, she is a bright and sweet child despite the jarring changes in her life. She latches onto Kippei as her surrogate "mommy" readily, and soon comes to see him as a combination parent/big brother/first crush.
The girls at school notice the obvious change in their resident playboy, and not all of them are happy about it. When one jealous fan girl learns that Yuzuyu is the focus of Kippei's attention, she starts picking on the little girl. Her vicious words wound Yuzuyu where it hurts the most, and Yuzuyu becomes withdrawn and anxious. Kippei notices the change in her but is unable to figure out what's wrong. Fortunately for him, help comes in the most unexpected of people: Kokoro, the one girl at school seemingly immune to his charms.
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