Synonyms for Relentless (same or very similar meaning)
WordNet sense 1 (not to be placated or appeased or moved by entreaty):
grim, inexorable, stern, unforgiving, unrelenting, unappeasableWordNet sense 2 (incapable of being placated):
implacableWordNet sense 3 (never-ceasing):
persistentFrom the ODE community, based on WordNetadd/edit