Formal:
* Confessor: (implies religious or spiritual trust)
* Counselor: (suggests professional advice-giving)
* Trustee: (emphasizes holding something in trust)
* Advisor: (implies giving guidance or counsel)
Informal:
* Best friend: (most common and casual)
* Close friend: (indicates strong bond)
* Soulmate: (implies deep emotional connection)
* Right-hand person: (suggests close collaboration and trust)
* Intimate: (emphasizes closeness and sharing)
* Companion: (highlights shared experiences)
* Close associate: (suggests a strong, often professional, bond)
Literary:
* Alter ego: (implies a close, almost interchangeable relationship)
* Confidant: (more formal, usually for a male)
* Bosom friend: (old-fashioned, suggesting deep intimacy)
The best synonym for "confidante" will depend on the context and the specific relationship you are trying to describe.