underdog vs unsuccessful person

underdog and unsuccessful person both are nouns.

Word	Noun	Adjective	Verb	Adverb
underdog	Yes	No	No	No
unsuccessful person	Yes	No	No	No

As nouns, unsuccessful person is a hypernym of underdog; that is, unsuccessful person is a word with a broader meaning than underdog:

underdog: one at a disadvantage and expected to lose
unsuccessful person: a person with a record of failing; someone who loses consistently

Other hypernyms of underdog include failure, loser, nonstarter.

underdog (noun)	unsuccessful person (noun)
one at a disadvantage and expected to lose	a person with a record of failing; someone who loses consistently