Maybe he's a hikikomori who's staying at that villa unbeknownst to them? And that's the reason the gang was sent there, to try and lure him outside. That'd be a twist.
I've been supportive of Miku since the beginning of the manga even though she isn't my favorite, but her character is getting quite annoying. She's gotten to be more of a pushover over the course of the manga, despite receiving so much support from her sisters. Please just let her have character...