Confidant
Etymology
From French confident, from Italian confidente, from Latin confidentem "trusting," from confidere "to trust fully" (com- intensive plus fidere "to trust"). The anglicized -ant spelling arose later. The core is fides — faith placed in someone to hold what you say.
noun · /KON-fih-dant/
The person you trust with your private thoughts and secrets.
What it names
A handle for a specific role rather than a general bond — the one you actually tell, who may or may not be your closest friend.
Example
Her sister was her confidant for everything except money, which she could only discuss with a friend outside the family.
Related idea
Distinct from friend: friendship is a broad two-way bond, while being a confidant is a one-directional trust — the safe repository for disclosure. Someone can be a treasured friend you would never confide in, and a confidant you are not otherwise especially close to.