unrelenting

Synonyms: bound bound and determined brick-wall ceaseless constant continual continuous cruel dead set on endless grim hanging tough hard-headed implacable incessant inexorable intransigent iron-fisted mortal perpetual persistent pitiless relentless remorseless rigid ruthless set steady stern stiff stiff-necked tenacious tough unabated unbending unbroken unflinching unremitting unsparing unwavering unyielding

Antonyms: compassionate flexible merciful relenting sympathetic