ungentle

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, weighty