That Spirit that plagues him resides in the Ring that he carries. That Ring will find him, no matter where it goes, but the Spirit just needs a host. When Ryou discovered the existence of that devil and how the Ring worked, he kept it.

He held onto it tightly, a reverse sort of protection -- for if the Spirit was with him, it couldn't take over another person. No matter how much his own body was abused or taken away from him -- no matter how many times he woke up with time lost, sometimes significant time -- he refused to release it. He took the burden onto himself so that others would be safe...

A burden that even many soldiers could never suffer.