grumbler vs unpleasant person

grumbler and unpleasant person both are nouns.

Word	Noun	Adjective	Verb	Adverb
grumbler	Yes	No	No	No
unpleasant person	Yes	No	No	No

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

Other hypernyms of grumbler include disagreeable person.

grumbler (noun)	unpleasant person (noun)
a person given to excessive complaints and crying and whining	a person who is not pleasant or agreeable