Kippei
Kippei is a very popular guy at school, especially with girls. At the beginning of the series, he seems to have about fifty girlfriends and a bit of an ego problem, so his family puts him in charge of Yuzuyu's care in an attempt to get him to be more mature.
Although initially Kippei isn't thrilled with the idea of having to take care of Yuzuyu, he does so without taking it out on her. He's never patronizing or condescending to her the way some people can be to young children, and it doesn't take long for him to grow very fond of Yuzuyu. He knows that she needs and misses her mother, so he makes an effort to reassure her and take good care of her. He makes mistakes, but Yuzuyu seems to really be happy under his care.
In general, Kippei is a pretty laid-back guy who doesn't let problems get to him, and that's what seems to allow him to concentrate on his new responsibilities. In the end, talking care of Yuzuyu does have the desired effect on him -- he immediately becomes too busy to think about girls all day and begins concentrate on making sure Yuzuyu doesn't suffer too much without her mother.