Let’s look at the clues. Mio and Midori both share the missing shadow, indicating that the two are linked somehow…meaning Midori is likely not a twin or something to that effect. More than likely another manifestation of Mio, which is why she can effectively replace Mio in everyone’s minds (my initial guess of the second personality, maybe?). Also, Mio wished for this to happen…she believed it would be best for this to happen. Meaning that Midori seems to be acting on her behalf, further supporting the same-person idea.That’s my speculation, I suppose. Midori seems quite malicious. I wonder if she’ll become more hostile as Riki tries to find Mio in the next episode. How does he expect to find her? Would it just be by learning more about her by looking around? Or will he constantly pester Midori? Bothering her doesn’t seem like it’ll be productive.