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John Rainbird is a Low Man/Can-Toi.
We know that most of Stephen King's works connect to each other, often in subtle ways. I propose that John Rainbird is one of the Low Men, minions of the Crimson King who track down escaped Breakers (psychics who are used to break the Beams holding up the Dark Tower) and recruit new ones. Right away you can see similarities. Furthermore, they drive garish, classic automobiles, and he drives a yellow Cadillac. And it's interesting that it's specifically yellow, since that's the main color associated with them (they first appeared in the story Low Men in Yellow Coats). Finally, he only agrees to help The Shop track down Charlie in exchange for gaining custody of her afterwards, because he wants to use her abilities. Use them to break the Beams, perhaps?