ringer vs impostor

ringer and impostor both are nouns.

Word	Noun	Adjective	Verb	Adverb
ringer	Yes	No	No	No
impostor	Yes	No	No	No

As nouns, impostor is a hypernym of ringer; that is, impostor is a word with a broader meaning than ringer:

Other hypernyms of ringer include fake, faker, fraud, imposter, pretender, pseud, pseudo, role player, sham, shammer.

ringer (noun)	impostor (noun)
(horseshoes) the successful throw of a horseshoe or quoit so as to encircle a stake or peg	a person who makes deceitful pretenses
a contestant entered in a competition under false pretenses
a person who is almost identical to another
a person who rings church bells (as for summoning the congregation)