Rui Saionji was a famous tennis player. She was Naru Osaka's childhood friend. They were very close, and Naru often referred to her as "onee-san" (お姉さん; meaning "older sister"). In anime episode 14 she was targeted by Nephrite for her energy. The Youma Tesuni possessed her tennis racquet, causing her playing style to become more aggressive. Eventually, however, Tesuni was defeated by Sailor Moon and Rui was restored to normal, with no memory of what had happened.

(Source: Project WikiMoon)