87 synonyms for relentless such as cruel, assiduous, cold-blooded, and cold-hearted. Relentless is an adjective, and its antonyms are short-lived, irresolute, and intermittent.
Assiduous, constant, continual, continuing, continuous, determined, diligent, dogged, endless, importunate, incessant, indefatigable, insistent, interminable, intransigent, lasting, never-ending, non-stop, obdurate, obstinate, patient, perpetual, persevering, persistent, pertinacious, purposeful, resolute, sedulous, single-minded, steady, stubborn, sustained, tenacious, tireless, unabated, unabating, unbroken, unceasing, undeviating, unending, unfaltering, unflagging, uninterrupted, unrelenting, unrelieved, unremitting, unstoppable, unswerving, untiring, and unwavering.
Cold-blooded, cold-hearted, cruel, fierce, grim, hard, hard-hearted, harsh, heartless, implacable, inexorable, inflexible, lacking compassion, merciless, obdurate, pitiless, punishing, remorseless, ruthless, severe, stony, stony-hearted, strict, unbending, uncaring, uncharitable, uncompromising, unfeeling, unforgiving, unmerciful, unmoved, unpitying, unsparing, unsympathetic, unyielding, and with a heart of stone.