Correct spelling, explanation: the word protester entered English in the times of Middle English from the Latin word protestari. Later the word evolved to the form protester, which we know today. Grammatically speaking, the word protester is a noun used in both formal and informal contexts.

Definition of protester:
1. noun – a person who openly shows his or her disagreement by shouting, using signs, etc.,
He has been an active political protester for years.

Collocations with protester:
Some well-known collocations include anti-government protester, unarmed protester, and protester against.
As an anti-government protester he spends a lot of his free time taking part in demonstrations.
Luckily, he was an unarmed protester so the police did not arrest him.
He is an active protester against racism in the world.

Incorrect spelling, explanation: many users of English spell the word protester as protestor and it is true that for many years both forms functioned equally. What’s more, some dictionaries state that the form protestor is right. However, when you google both forms, it is clearly visible that protester has much more results than protestor and most linguists agree that the form which we should use as the correct one is protester.