southpaw vs doer

southpaw and doer both are nouns.

Word	Noun	Adjective	Verb	Adverb
southpaw	Yes	No	No	No
doer	Yes	No	No	No

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

Other hypernyms of southpaw include actor, worker.

southpaw (noun)	doer (noun)
a person who uses the left hand with greater skill than the right	a person who acts and gets things done
a baseball pitcher who throws the ball with the left hand