relentless vs stern

relentless is not a noun while stern is a noun.

Word	Noun	Adjective	Verb	Adverb
relentless	No	Yes	No	No
stern	Yes	Yes	No	No

As adjectives, relentless and stern are synonyms defined as:

relentless and stern: not to be placated or appeased or moved by entreaty

Other synonyms of relentless include grim, inexorable, unappeasable, unforgiving, unrelenting.

relentless (noun)	stern (noun)
	the rear part of a ship
	the fleshy part of the human body that you sit on

relentless (adjective)	stern (adjective)
never-ceasing	severe and unremitting in making demands
not to be placated or appeased or moved by entreaty	not to be placated or appeased or moved by entreaty
	of a stern or strict bearing or demeanor; forbidding in aspect
	severely simple