Ayu Anjou

The second girl who could be Yousuke's sister. Compared to Mayu's true age, she is only a few months younger. She is a Skilled basketball player. Ayu is also very protective of Mayu, Her father remarking "She is more of a watchdog than a sister" upon Yousuke introduced to her. Although she is kind towards Yousuke, she does not hesitate to attack him when she thinks he is being a pervert. In chapter two, she reveals that Yousuke is just like her brother at the orphanage, but she doubts that he really is.

(source: Wikipedia)