temperament; disposition; personality
Refers to one's inherent nature or way of thinking, often used in contexts of compatibility or mutual understanding. Common in phrases like 気心が知れる (to know someone's personality well).
Having worked together for many years, I know his temperament well.
気心の知れた仲間と旅行するのは楽しい。
Traveling with friends whose personalities you know well is enjoyable.
I've worked with this colleague for years, so we really know each other's true nature.
He and I are so alike in temperament that traveling together never wears me out.