expression, noun-modifying (irregular)
Describes a friend, colleague, or companion with whom one can be completely at ease, sharing true feelings without pretense. Used prenominally (before a noun).
彼は気心の知れた友人だ。
He is a trusted friend I can be myself with.
気心の知れた仲間と旅行するのは楽しい。
Traveling with close companions who know you well is fun.
信頼できる means 'reliable' or 'trustworthy' and can apply to people, systems, or information. 気心の知れた is specifically about personal relationships where one's true nature is understood.
A fixed expression combining 気心 (one's true feelings and nature) and 知れた (the perfective form of 知れる, 'to be known'). Literally 'one's heart and mind are known', it describes a relationship of deep mutual understanding.