"People with a strong mental power" is "멘탈갑"(men-tal-gap) in Korean.
멘탈갑 (Mental Gab), which means "spirit, mental strength" compound word of the English word Mental(spirit) ,and 갑 (Gab) which means "high, top, best". It has a strong mental power that does not lose to any situation and challenges; it is possible to use for the people who looks strong, and without having worries about the little things, and go to the right ways. It often used to such as athletes.