Relentless
Constant
Synonyms: Persistent, constant, continuous, lasting, steady, unabated, unbroken, incessant, unceasing, unending, unremitting, unrelieved, unfaltering, untiring, unswerving, persevering, resolute, dogged, tireless, patient, assiduous, tenacious, insistent, stubborn, obstinate, continuing, continual, non-stop, never-ending, uninterrupted, unabating, interminable, unstoppable, endless, perpetual, unrelenting, sustained, unflagging, unwavering, undeviating, determined, purposeful, single-minded, indefatigable, diligent, sedulous, pertinacious, importunate, intransigent, and obdurate.
Fierce
Synonyms: Harsh, fierce, severe, punishing, merciless, ruthless, unsparing, hard-hearted, stony-hearted, with a heart of stone, cold-hearted, unfeeling, unsympathetic, lacking compassion, inflexible, uncompromising, unyielding, inexorable, grim, cruel, strict, remorseless, pitiless, unmerciful, heartless, hard, stony, cold-blooded, unforgiving, uncaring, uncharitable, unpitying, unbending, obdurate, unmoved, and implacable.
Example Sentences
The storm was relentlessly fierce.
Her efforts were relentless today.
They faced relentless financial struggles.
He showed relentless determination throughout.