A trusted or close (person); someone whose temperament and mind you know well. A non-standard variant of 気心の知れた.
気の知れた友人となら、率直に意見を言い合える。
With a close, trusted friend you can speak frankly with one another.
彼は気の知れた同僚なので、頼みごとをするのも気楽だ。
He's a colleague I know well, so it's easy to ask him for favors.
気の知れた仲間だけを招いて小さな会を開いた。
I held a small gathering inviting only close, trusted companions.