If you describe someone as imperturbable, you mean that they remain calm, even in disturbing or dangerous situations. Thomas, of course, was cool and aloof and imperturbable. remaining calm and unworried in spite of problems or difficulties
not easily perturbed or excited or upset; marked by extreme calm and composure; "hitherto imperturbable, he now showed signs of alarm"; "an imperturbable self-possession"; "unflappable in a crisis"