I was somewhat annoyed in the first few episodes, the way the characters acted felt very forced (probably my culture-shock from regular to shoujo anime >_>). That feeling went away after a couple of episodes and I actually started caring for the characters.
The story is about a private school for girls, who have their own version of the Senpai / Kohai system, the "soeur" (French for sister) system, a much closer system in which the younger "sister" helps the older one out with minor tasks, and the older one takes the younger in her care. It follows Yumi, a freshman, as she gets involved with Sachiko, an upperclassman with a lot of status on the school.
It takes some time for this series to pick up, but once all relations have been settled, it becomes a very enjoyable, character-driven shoujo-ai anime.