guardian vs fireman

guardian and fireman both are nouns.

Word	Noun	Adjective	Verb	Adverb
guardian	Yes	No	No	No
fireman	Yes	No	No	No

As nouns, fireman is a hyponym of guardian; that is, fireman is a word with a more specific, narrower meaning than guardian:

guardian (noun)	fireman (noun)
a person who cares for persons or property	play in which children pretend to put out a fire
	a member of a fire department who tries to extinguish fires
	a laborer who tends fires (as on a coal-fired train or steamship)
	a pitcher who does not start the game