I think Rei is the host because he seems most aware of the situation, and from what we learned about him throughout the chapters.
1) antisocial: in CH. 1 all the coworkers say he's hard to work with because he doesn't socialize with anyone. With all his personalities they seem very social, but Rei even with yuuta keeps his guard up and continously pushes him away with DID.
2) he always seems aware of when he's going to transform like with itsuki he went home immediately, and his statement about his female alter being integrated felt as though he took a big loss and relief at the same time. Also, kusaka didn't feel affected by it.
3) Rei's behavior seems more characteristic of someone who has suffered abuse and abandonment, but the same can be said about kasuka. Reasons being if you've suffered it before it can go multiple ways but most commonly you avoid people in fear you may experience it again, or do everything to keep them.
However it also leaves the question who the mysterious garden alter is because he too was aware of itsuki, or what if it is itsuki who is the host?