Confidant (noun)

A person with whom one shares a secret or private matter, believing one will not be betrayed or exposed.

Origin:

Early 18th century: from french confident, from confidenter "to trust", from latin confidere "have full trust in".

Examples:

1. He confided in her as his closest confidant.
2. She was his confidant throughout his difficult marriage.
3. He told his confidant about the situation he was going through.
4. She was the only confidant he had during the scandal.
5. He trusted her as his confidant and shared all his personal thoughts.