Wow, the concept of this game sounds interesting! The chararcter design looks good but unfortunately the seiyuu cast doesn't particularly appeal to me.
But I think it'll be interesting to give this game a try since I've not really played any otome games with mystery/thriller elements... thought somehow this is reminding me of Persona 4 which I'm playing now. *laughs* (Probably 'cos that game has dating sim elements and a mystery theme as well.
And I'm just going to be random and guess that Mio might be the killer since he looks clever enough to think up murder strategies, yet nice enough that the game makers might want to trick us with him.