BanG Dream!: Star Beat

Toyama Kasumi, a first year high schooler with a shy personality, can no longer sing and express her emotions ever since she was made fun of her singing her favorite song. She is lonely and could not make friends even after entering high school. The only time she could show her emotions is through an exchange diary on her desk with a mysterious person named "Saaya" (サアヤ). Saaya seems to attend classes at a fixed time.