a person or thing that demonstrates.

Also, demonstrant. a person who takes part in a public demonstration, as by marching or picketing.

a person who explains orteaches by practical demonstrations.

a person who exhibits the use and application of (a product, service, etc.) to a prospective customer.

the product, device, machine, etc., actually used in demonstrations to purchasers or prospective customers

They sold the demonstrator at half price.

noun

agitator , boycotter , disrupter , dissenter , marcher , objector , picketer , radical , revolter , revolutionary , rioter , striker , troublemaker