WordNet sense 1 (stubbornly unyielding):

dogged, pertinacious, tenacious, unyielding, dogged

WordNet sense 2 (tenaciously unwilling or marked by tenacious unwillingness to yield):

obstinate, stubborn, unregenerate

WordNet sense 3 (harshly uninviting or formidable in manner or appearance):

forbidding, grim

WordNet sense 4 (disagreeable to the senses, to the mind, or feelings ):

unpleasant

WordNet sense 5 (showing a brooding ill humor):

dark, glowering, glum, moody, morose, saturnine, sour, sullen

WordNet sense 6 (having an irritable and unpleasant disposition):

ill-natured