Confidant
The confidant (feminine: confidante, same pronunciation) character is usually someone the lead character confides in and trusts. Typically, these consist of the best friend, relative, doctor or boss. The confidant character is most common in romantic comedy film but often appear in other genres.
Examples of a confidant character, would be Rob Reiner to Bruce Willis in The Story of Us, Kevin Pollak to Matthew McConaughey in The Wedding Planner or Charles S. Dutton to Johnny Depp in Nick of Time.