im·per·turb·a·ble
/ˌimpərˈtərbəb(ə)l/
adjective
unable to be upset or excited; calm.
"an imperturbable tranquility"
synonyms: self-possessed, composed, collected, calm, cool, calm, and collected, as cool as a cucumber, coolheaded, self-controlled, poised, tranquil, serene, relaxed, easygoing...