@TheOneToRule Mika is implied to also be autistic and non-verbal as a child. When overwhelmed by it she would shut down, go limp and "become all creepy". Her mother would rationalize it as "a demon possessing her", and claim that "she used to be such a bright child", things that parents of autistic children tend to say when in denial of that fact. Mika "breaks through" while in isolation and starts speaking, which her mother thinks means she's "cured", but the way she was mistreated saddled her with a lot of other issues that she's struggling to deal with (also a sadly common thing for autistic people who grew up non-verbal).
This is why when she sees Nao she thinks of maybe talking to her about it, but is too afraid to because to her acquiring verbal skills was part of a tragedy. In the shorter chapters set in the future Nao can speak, but Yui is nowhere to be seen, implying a similar tragedy maybe?