Imperturbable

Adjective

 * 1) not easily perturbed, upset or excited
 * 2) calm and collected, even under pressure

Thesaurus
balanced, bovine, collected, complacent, composed, cool, dispassionate, dull, even-tempered, fast, fiducial, firm, firm as Gibraltar, impassive, in equilibrium, inexcitable, inirritable, nonchalant, patient, predictable, reliable, secure, self-satisfied, smug, solid, sound, stable, steadfast, steady, stoic, stolid, substantial, unaffected, undisturbable, unflappable, unflinching, unirritable, unmoved, unnervous, unpassionate, unshakable, untouched, unwavering, well-balanced, without nerves