insouciant

blasé (also blase), breezy, cavalier, nonchalant

casual, easygoing, informal, laid-back, low-pressure, relaxed, unfussy

halfhearted, lukewarm, tepid

aloof, cold, numb, remote, unemotional

callous, hard-hearted, insensitive, unfeeling

calm, cool, detached, dispassionate

careless, heedless, mindless

impassive, impervious, phlegmatic, stoic (or stoical), stolid

lethargic, listless

unimpressed