confidant

A confidant is the person you tell the thing you have told nobody else. The word comes from confide, and the whole of it is trust: you hand something over and it stays handed over. A friend is someone you like; a confidant is someone you are safe with, which is rarer. Confidante is the same word spelled for a woman. In the animation, the listener's head is a safe that swallows a secret and stays shut.