significant other vs doer

significant other and doer both are nouns.

Word	Noun	Adjective	Verb	Adverb
significant other	Yes	No	No	No
doer	Yes	No	No	No

As nouns, doer is a hypernym of significant other; that is, doer is a word with a broader meaning than significant other:

significant other: a person (not necessarily a spouse) with whom you cohabit and share a long-term sexual relationship
doer: a person who acts and gets things done

Other hypernyms of significant other include actor, worker.

significant other (noun)	doer (noun)
a person (not necessarily a spouse) with whom you cohabit and share a long-term sexual relationship	a person who acts and gets things done