impassible

cold, cool, dispassionate, unmoved

calm, collected, composed

imperturbable, unflappable

reserved, reticent, taciturn

bland, blank, deadpan, dry, empty, expressionless, inexpressive, stone-faced, straight-faced, vacant, wooden

enigmatic (also enigmatical), impenetrable, inscrutable

aloof, bloodless, detached, indifferent, insensible, unconcerned, unsentimental

impersonal, objective, unresponsive

hard-hearted, pitiless, unfeeling

inconsiderate, thoughtless