[@DeathXtheXTree]
Yuu can speak, she just chooses not to until a certain event happens. I notice that Karu is supposed to be aloof unless interested, so maybe she just wonders what goes on in the silent sister's mind? Could be a valid reason for them to interact.

Hopefully Yuu can dissuade Karu from even [I]thinking[/I] that taking innocent lives is an option, mostly through kindness... and alot of snark if that doesn't work.

...I can't believe I just took the 100th OOC post to talk about a mute girl.

Edit: Just remembered. Ai is a thing. She would definitely notice if Yuu started thinking as if she was having a mental conversation with someone, and want to know who it was her sister was talking to [S]and how fast she can make them stop[/S]