runner-up vs competitor

runner-up and competitor both are nouns.

Word	Noun	Adjective	Verb	Adverb
runner-up	Yes	No	No	No
competitor	Yes	No	No	No

As nouns, competitor is a hypernym of runner-up; that is, competitor is a word with a broader meaning than runner-up:

Other hypernyms of runner-up include challenger, competition, contender, rival.

runner-up (noun)	competitor (noun)
the competitor who finishes second	the contestant you hope to defeat