even, even-keeled, steady, well-adjusted, well-balanced

imperturbable, nerveless, unflappable, unshakable

centered, disciplined, equable, self-contained, self-controlled

affable, breezy, devil-may-care, easygoing, happy-go-lucky, laid-back, loosey-goosey, mellow

carefree, nonchalant, unconcerned

assured, confident, self-assured

aloof, detached, dispassionate, indifferent

bovine, impassive, phlegmatic, sober, stolid

relaxed, relieved, tranquilized (also tranquillized)