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