A walk-in is an entity who has come into a human body as a kind of soul exchange.
Most souls come into the human body at birth.
Normally, this soul exchange is prearranged and agreed upon by both soul entities. I find this a very interesting concept. Seeing the way that free will runs rampant in this place we call Planet Earth, it's been mostly destructive. Where I'm from there is no free will. It does not restrict freedoms.