Usagi Drop

From Yen Press:

Going home for his grandfather's funeral, thirty-year-old bachelor Daikichi is floored to discover that the old man had an illegitimate child with a younger lover! The rest of his family is equally shocked and embarrassed by this surprise development, and not one of them wants anything to do with the silent little girl, Rin. In a fit of angry spontaneity, Daikichi decides to take her in himself! But will living with this overgrown teenager of a man help Rin come out of her shell? And hang on, won't this turn of events spell doom for Daikichi's love life?!

Note: Was adapted into a live-action movie in 2011.