From Intercross:

Tooru Miyamoto, is a kind, conservative "sister" much like the Virgin Mary. An accidental yet unforgettable encounter with someone's fingertip's touch made her want to touch it one more time. But, Tooru had a secret. Aside from being called the "Immoral Sister," she is also called the "Crying Mary," the "Green Monster," the "Shadow Sister," and the "Capricious Pianist."