Mayu Anjou

One of the two girls who could have been Yousuke's sister. She is skilled at the violin and plays it at a professional level. She is really close to Ayu and likes that Yousuke tries to be a big brother to her. Of the two girls, she is the only one that calls Yousuke "onii-chan" (lit. "big brother"). The manga also shows that she is more caring than Ayu towards Yousuke.

(source: Wikipedia)