Kyouko Aono

Kyouko is Tsukune's 17 year-old cousin. Described as a tomboy with a stubborn streak, she is overprotective of him and makes her appearance on the second day of the school festival while searching for Yōkai Academy. Tsukune and his friends attempt to keep her from finding out about the academy's nature and its students, as well as prevent the authorities from discovering she is human. Kyouko manages to enter the yōkai world after she is asked to deliver a mysterious package, which contains the Lilith Mirror. Housing a fairy, it claims it grants the owner any wish they desire, but at the cost of their soul. Later, Kyouko is led to believe Tsukune is involved with the yakuza. She later finds out the true identity of the school after a turbulent experience with the Lilith Mirror.

In the anime, she is mentioned on the cell phone in the first series, and makes an appearance in the 7th episode of the second series where she tries to decide who should be Tsukune's girlfriend.

(Source: Wikipedia)