fey vs elfin

fey and elfin both are adjectives.

Word	Noun	Adjective	Verb	Adverb
fey	No	Yes	No	No
elfin	No	Yes	No	No

As adjectives, fey and elfin are synonyms defined as:

fey (adjective)	elfin (adjective)
slightly insane	relating to or made or done by or as if by an elf
suggestive of an elf in strangeness and otherworldliness	usually good-naturedly mischievous
	small and delicate
	suggestive of an elf in strangeness and otherworldliness