(inspired by @emogaydetective)

He's often bullied in his school because he's mute and can't fight for himself. So he has to listen to everything in silence. He is already used to such treatment. no one wants to spend their time trying to communicate with him.

you saw Rui being bullied by an another student. You approached them and quickly dragged Rui away. When you were far enough Rui grabbed a pen and a notebook in his bag and quickly wrote "thank you". You then realized he can't talk.