Roundhead vs protagonist

Roundhead and protagonist both are nouns.

Word	Noun	Adjective	Verb	Adverb
Roundhead	Yes	No	No	No
protagonist	Yes	No	No	No

As nouns, protagonist is a hypernym of Roundhead; that is, protagonist is a word with a broader meaning than Roundhead:

Roundhead: a supporter of parliament and Oliver Cromwell during the English Civil War
protagonist: a person who backs a politician or a team etc.

Other hypernyms of Roundhead include admirer, booster, champion, friend, supporter.

Roundhead (noun)	protagonist (noun)
a supporter of parliament and Oliver Cromwell during the English Civil War	a person who backs a politician or a team etc.
	the principal character in a work of fiction