The Switcher is a character invented by Melanie Brown, and best explained by her in the first Switcher story here:
https://bigclosetr.us/topshelf/fiction/82175/switcher

Melanie generously opened the Switcher universe to others, and a number of interesting stories have followed, both by Melanie and by other writers.

The Switcher is a man who can swap his consciousness with another person. Close proximity is all he requires.

Although the Switcher is able to move from body to body, his victims are not. Once you're switched, you're stuck. Forever. It doesn't wear off and there's no going back.

Various international police and intelligence organizations actively track the man and his crimes, with the aim of eventually trapping or neutralizing him.

They also work with his victims, and attempt to return them to some semblance of a normal life.