Sumi is cool but... What degree of introversion do you need to reach to not use your mouth to talk to someone. I'd be fine it were to be in public only, or something. But even when they're alone on the beach she just won't use her voice. Did the author want to give off a Koe no katachi vibe? Well it failed, bad writing.
But it's ok, Sumi is cool and the MC didn't keep a 100% perv face for the whole chapter, so I'm good with it.