relentless
1 cruel, fierce, grim, hard, harsh, implacable, inexorable, inflexible, merciless, pitiless, remorseless, ruthless, uncompromising, undeviating, unforgiving, unrelenting, unstoppable, unyielding
2 incessant, nonstop, persistent, punishing, sustained, unabated, unbroken, unfaltering, unflagging, unrelenting, unrelieved, unremitting, unstoppable
Antonyms
1 compassionate, forgiving, merciful, submissive, yielding