What do you call the person sitting behind the driver on a bike?

Pillion is the right word for the person too. Source Wikipedia

A passenger in this seat is said to "ride pillion" or may themselves be referred to as a "pillion".

Or you can say 'pillion rider' as in these news articles.

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".

If the motorcycle passenger is a girl, you might consider back warmer

back warmer: a girl on the back of your motorcycle

Total Motorcycle

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.