I think I mixed up the chars and thought it was one of the two main chars and not one of the important side ones... Sorry, but she does have a sister. Problem with not watching the series in a while...
Anyhow, this girl is from a very rich family and a friend of the younger of the sisters the story is about. Sorry for mixing that up...
The series is a drama and slice of life that has got some flak for being unrealistic, which always confuses me when people say about anime...