relentless

Synonyms: adamant bound bound and determined dead set on determined dogged ferocious fierce go for broke grim hang in hang-tough hard harsh implacable inexorable inflexible inhuman mortal obdurate pitiless remorseless rigid rigorous ruthless single-minded stiff stop at nothing strict stringent unappeasable unbending uncompromising undeviating unflinching unforgiving unrelenting unstoppable unyielding vindictive

Antonyms: kind merciful sympathetic understanding

Synonyms: incessant nonstop persistent pertinacious punishing sustained tenacious unabated unbroken unfaltering unflagging unrelenting unrelieved unremitting unstoppable

Antonyms: ending intermittent stopping