Mio Kouzuki

Mio is a mute underclassman of Kōhei's with abundant expressions. Since she cannot speak, she uses a sketchbook to carry on written conversations. She belongs to the theater club as a junior of Yukimi Miyama. While she performs non-speaking parts, she handles her own role reliably. She is quite a dojikko, an anime stereotype for klutzy cute girls. Other than the sketchbook she uses, she has an older sketchbook which she treasures greatly.

From Wikipedia.