Khon Jai Yen

khon jai yen /kʰon dʒaɪ jen/ (คนใจเย็น)

noun

Thai expression, literally "cool-hearted person"

1. A calm person who stays level-headed under stress.
2. A patient, even-tempered person who is not easily upset.
3. Someone who knows that true strength doesn’t need to be loud.

You should start here, btw.