Kousagi is Chibiusa's little sister from an alternate reality. Kousagi only appears in the mini manga at the end of Sailormoon Artbook VI, the materials collection. Kousagi is flighty, flaky, and klutzy, but incredibly cute. Chibiusa cares about her, but she also annoys Chibiusa much in the same way Usagi annoys her. Why can't she be more responsible and level-headed, after all. God save us all if Kousagi is left to defend the world by herself. In the end, Kousagi is really too sweet and good-natured for anyone to dislike for long (even her sister, who is forced to put up with her all the time) and Chibiusa does love her, although she'd probably only admit it under duress.
It's a case of not being able to hate on something that cute and dumb for very long, especially if she's your little sister.