Here are some links to previous pages for anyone who may be wondering about Demetrius’ specific condition.

Panel 4 of Page XIII – Ash explains.

Panel 1 of Page XXXIV – Cass uses inaccurate wording on accident here. It’s easy for Demetrius’ friends to forget he can speak because he only speaks to the people closest to him, and in private.

Last panel of Page CCXXIX – Technically Demetrius’ first word balloon in the comic.

Panel 6 of Page CCLXXXVII – Ash vaguely describes what it’s like when Demetrius speaks.