Not sure, maybe the host, the walk-n, both, the world, the universe, etc.
"While the New Age belief system about walk-ins claims that these transitions can't occur involuntarily and that no soul walks into another's body without reason, the behaviour of some "new" people indicates that it may not always be so.
In classical cases, the change is immediately apparent. However, in cases where the "new soul" has enough information to take up the life of the "previous occupant" seamlessly, it may take weeks or months before a walk-in notices, or comes to believe, that a transition has indeed occurred. Occasionally, the "old self" returns after a period of months or years, and either the "new self" departs, or they coexist and may try to integrate into a single being, or work out a means of cooperation and live as two persons in one body."
That is quoted, about one third down the page, from:
http://www.crystalinks.com/walk_ins.html
It's one viewpoint. I enjoy reading and hearing about many on these topics.