I know pillion is a seat behind the rider of a horse or motorbike etc.
I would like to know the word for the person sitting on pillion.
If you're in casual conversation, you can just call them the person in back, but if you need to be technical, they can be called the
stoker, navigator, tailgunner or rear admiral.
Those words are fantastic, IMHO, but they are esoteric to most casual bike riders. Best to stick with "front/rear rider" or "person in front/back".
In the US, I'd refer to that person as "riding shotgun", although that usually is in reference to riding beside the driver in a car. It's origins appear to come from riding stagecoaches.
It's a more recent phrase than riding pillion, per ngram. Ngram does support that usage "riding pillion" is more common in British English than American English.
Much, much less common is the term "Wookie", originating from the film "Fanboys". The passenger assumes the role of Chewbacca, the Wookie, implying the driver is Han Solo, and the vehicle in question is the Millenium Falcon.