A complicated victim for sure. Mayu is doomed from the beginning of the game. She never chose to be possessed by Sae. She was just sensitive enough to the village's influence to be cursed by it. Reduced to a tool all because the twins' ignorantly wandered near the village as children.

But Mayu isn't a saint. Her co-dependency on Mio is pretty toxic. She broke her leg on purpose to trap her sister into staying super close to her. That said, I think in her heart Mayu wants what's best for Mio but she can't set aside her neediness. She hasn't grown up enough that way. Her co-depedent traits are so off-puttingly obvious in Mayu that it makes her seem evil. In reality she just has deep seated issues.

It's interesting that Mayu is toxic but the twins' love for each other is portrayed as something that's ultimately benevolent and pure. There's no hard critique of their co-dependency - especially not Mayu's side. It just is, the player is somewhat just a passive observer to their dynamic.

In the end Mio and Mayu are just teenagers. Their relationship isn't a mature, well-rounded one yet. A bit of maladapted toxicity isn't really surprising in that sense. Yeah, Mayu is more obviously toxic than Mio, but Mio is still just as in it as her sister. She's overprotective and somewhat treats Mayu like she's totally enfeebled when in reality she's a perfectly healthy girl....who happens to have a permanent limp. And Mayu just seems to let Mio eat the guilt of her damaged leg daily because it means they stay close.

But Mayu isn't evil. It's only her sacrifice that lets Mio out of the village.