grim

bleak, cold, hostile, inhospitable, inimical, unfriendly, unsympathetic

adamant, bound, determined, firm, intent, purposeful, resolute, resolved, steadfast, unflinching

fixed, hard, hardened, hardheaded, immovable, implacable, inflexible, ironhanded, mulish, obdurate, obstinate, rigid, self-willed, set, stiff, stubborn, unbending, uncompromising, unrelenting, unyielding, willful (or wilful)

immutable, unchangeable

black, cheerless, dark, gloomy, glum, joyless, melancholic, moody, morose, sulky, sullen, surly

brooding, grave, humorless, melancholy, serious, sober, sobersided, solemn, somber (or sombre), staid, unsmiling

austere, bleak, comfortless, discomforting, forbidding, inhospitable, spartan, uncomfortable

rigorous, strict, stringent

agonizing, heartbreaking, heartrending, painful, wretched

crushing, grinding, overwhelming, wearing

insufferable, insupportable, intolerable, unbearable, unendurable

harrowing, tortuous

bad, disagreeable, hostile, unfriendly, unpleasant

persevering, persistent, tenacious

hard, hardheaded, headstrong, intractable, mulish, obdurate, opinionated, peevish, pertinacious, perverse, pigheaded, self-willed, stubborn, uncooperative, willful (or wilful)

merciless, ruthless, unforgiving

argumentative, assaultive, bellicose, belligerent, brawly, combative, confrontational, discordant, disputatious, feisty, gladiatorial, militant, pugnacious, scrappy, warlike

bare-knuckle (also bare-knuckled), blood-and-guts, in-your-face, take-no-prisoners

menacing, threatening

bestial, brute, inhuman, inhumane

barbaric, uncivilized, wild

heartless, implacable, merciless, pitiless, relentless, ruthless, unrelenting, wanton

bloodthirsty, bloody, homicidal, murdering, murderous, sanguinary, sanguine

rapacious, ravenous, voracious