fiend vs unpleasant person

fiend and unpleasant person both are nouns.

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

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

Other hypernyms of fiend include disagreeable person.

fiend (noun)	unpleasant person (noun)
a cruel wicked and inhuman person	a person who is not pleasant or agreeable
a person motivated by irrational enthusiasm (as for a cause)
an evil supernatural being