confidant
https://gyazo.com/2d1ff1f465d8ed212f8ab8e1d91c0b91
noun (feminine confidante pronounced same)
a person with whom one shares a secret or private matter, trusting them not to repeat it to others:
(何でも打ち明けられる)親友, 腹心の友.
e.g. a close confidante of the princess.
ORIGIN
mid 17th century: alteration of confident (as a noun in the same sense in the early 17th century), probably to represent the pronunciation of French confidente ‘having full trust’.