confidante
知己的女友
Con·fi·dant 277), n. masc., Con·fi·dante n. fem. One to whom secrets, especially those relating to affairs of love, are confided or intrusted; a confidential or bosom friend.
You love me for no other end
Than to become my confidant and friend;
As such I keep no secret from your sight. --Dryden.
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confidante
n : a female confidant