confidant
n. C1 Advanced US //ˈkɑnfəˌdɑnt// UK //kˈɒnfɪdˌænt// con·fi·dant
n. a person you trust completely and share your private thoughts with. You usually choose this person because you know they will keep your secrets.
n. a person to whom one's private secrets or thoughts are confided. Often implies a relationship of mutual trust and discretion.
She shared her secret with her closest confidant.
The CEO met with his most trusted confidant to discuss the company's financial problems before the board meeting.
In a world where information is currency, having a true confidant is rare; most people fear that even their most intimate revelations might eventually be used against them.
From French confident.