enemy vs mortal enemy

enemy and mortal enemy both are nouns.

Word	Noun	Adjective	Verb	Adverb
enemy	Yes	No	No	No
mortal enemy	Yes	No	No	No

As nouns, mortal enemy is a hyponym of enemy; that is, mortal enemy is a word with a more specific, narrower meaning than enemy:

enemy (noun)	mortal enemy (noun)
an opposing military force	an enemy who wants to kill you
any hostile group of people
an armed adversary (especially a member of an opposing military force)
a personal enemy