Equable: Even and steady; serene; does not vary; not easily irritated
Example: His equable manner made all those around him instantly comfortable.
I like this from Vocabulary.com:
An equable person isn’t moody. You wouldn’t expect him or her to fly into a rage one minute and be humming a happy tune the next. Instead, someone who is equable takes things in stride — the good, the bad, and the ugly, with a smile and the occasional reminder that “this too shall pass.”
Equable comes from the Latin word aequus meaning “equal”. From aequus, English also derives equilibrium, inequality, inequitable, equanimity, equivalent, adequate, equinox, equivocal, equivocate, and equal.
Synonyms include: composed, easygoing, level-headed, serene, tranquil, unruffled, or my personal favorite… sang-froid.