noun

a person with whom one shares a secret or private matter, trusting them not to repeat it to others.

a close confidante of the princess

close friend,bosom friend,best friend,close associate,companion,crony,intimate,familiar,second self,mentor,adviser,counsellor,alter ego,consigliere,chum,pal,buddy,main man,mate,oppo,mucker,bezzie,fidus Achates