I wish the little sister was the protagonist, her body type is much more appealing for this kind of story, but on the other hand there is not much room of development for her because she already knows and gets what she wants, so I get why Mio is the main character.
Still, Sarasa is fantastic, the whole walk sequence was the best, how she awakened as Mio said more woofs, the fact that for Mio the collar was still there in the train and Sarasa knew.
So when is someone gonna point out all the masochists character names start with M and the sadists with S?