Concept Idea

Think Yugi and Atem or Yuma and Astral but without an item conduit. The body is the conduit till later in the rp until one is found.
After the events of Don Thousand, Shawn's physical body was destroyed but the soul had turned into numeron code.  Shawn or rather Matthew Chaise had chosen to leave the code at some point but needed a host, someone who could hold his soul and by extension, memories, habits, ETC.

More or less, Shawn/Matthew is now apart of Alex's soul giving him a version of more Atem in concept. All his dueling and life skills go into the boy causing his mind to take a back seat and watch as when needed and when not, they are a voice in the back of his head telling him what he should and shouldn't do as if they are Alex's own.

Later on, I want less of a parasitic relationship and more of a partnership between them since Shawn/Matthew is forming the boy's body into a proper host to handle his soul. Which would lead to his abilities as well which will be basically the watered down version of them unless it's a complete transformation akin to Atem and Yugi's