愛してるぜベイベ [Aishiteruze Baby] Series by Yōko Maki

Kippei Katakura is a 17-year-old playboy who spends his time chasing girls, careless of their feelings. But when his 5-year-old cousin Yuzuyu comes to live with his family after her mother's sudden disappearance, Kippei is put in charge of taking care of her. As Kippei gets to know Yuzuyu and starts to understand how she feels, he also begins to realize that all girls were like Yuzuyu once...

Yuzuyu has a new classmate, Aya, whose older sister takes care of her just like Kippei takes care of Yuzuyu. Unfortunately, Aya and Yuzu overhear Aya's sister complain to Kippei about having to take care of a small child. Kippei starts spending time with Aya and her sister to help out, but Kippei's girlfriend Kokoro is the one who needs Kippei the most...

Kippei Katakura is a 17-year-old playboy. When his 5-year-old cousin Yuzuyu comes to live with his family after her mother disappears, he is suddenly in charge of taking care of her. Kippei's main occupation changes from tending to his high-school girlfriends to picking Yuzuyu up from kindergarten and making her lunch. Soon the two are getting along like peas in a pod, but the girls at his high school miss their "quality time" with Kippei.