mainstay vs protagonist

mainstay and protagonist both are nouns.

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

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

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

mainstay (noun)	protagonist (noun)
the forestay that braces the mainmast	a person who backs a politician or a team etc.
a central cohesive source of support and stability	the principal character in a work of fiction
a prominent supporter