@MK-95-
"ppearances can be deceptive, I'll agree with you on that point. However, we're given no reason whatsoever to doubt Mashuu's grandmother."
Yes, I know that, but Satoko isn't us, the readers.
"My problem lies with the circumstances she seeks to use as her excuse for entering his life. To her, it'd be ideal if he's being neglected or abused and deep down in her heart, she's probably hoping that's the case... That's really fucking toxic and I can't get behind her when she's thinking like that. "
Aaah, everything these days is "toxic".
Toxic this, toxic that.
People are overusing this word.
Satoko have no ill intentions, she is just conflicted. She don't want Mashuu's life to be "in turmoil", I'm 100% sure of it. However because she is so concerned with the turmoil that will happen in both of their lives if she gives in to Mashuu's desires she is desperately searching for a way in. As I said before (sorry for all those spelling mistakes), she is stopping to think about the situation she is and being influenced by this in the wrong way. Because she can't stop thinking about herself as "an adult" and about Mashuu as "a child", she is trying to hard too be an adult. "An adult must be responsible and protect the child", because she can't allow herself to have an honest and equal relationship with Mashuu as woman and man she is forcing herself to think this way.
The devil was this new friendship of Mashuu with her sister, and the realization of how little she really knows about the kid.
"What I don't know and what he didn't tell me, or worse, what he hid from me?".
See, is more complicated but perfectly understandable and you don't need to resort to "toxic" logic.
About Nao, "of course" she will play the role of "villain".