Ruri Kujo is the elder sister of the series' protagonist Kazuya Kujo.
Like Kazuya, she grew in fear of her father and elder brothers, but mostly because of their slovenly behavior. She is kind to Kazuya, and believes he was better born as a girl.
She was first mentioned in Gosick episode 9, where she sent Kazuya a shopping list. Among the items in that shopping list was a glass replica of the Blue Rose, the missing prized gem of Sauville. Shopping for the items she asked for was what led Kazuya to find the genuine Blue Rose being auctioned in a department store in Saubreme.
Her first official appearance in the anime was in Gosick episode 12. She appears in Kazuya's flashback as he reminisces memories of summer in Japan.