stern vs relentless

stern is a noun but relentless is not a noun.

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

As adjectives, stern and relentless are synonyms defined as:

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

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

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

stern (adjective)	relentless (adjective)
severe and unremitting in making demands	never-ceasing
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