Here are some options with varying shades of meaning:
Formal & Respectful:
* Fellow: Can be used for a companion or associate, often implying shared interests or goals.
* Companion: This emphasizes the companionship aspect of the relationship.
* Comrade: Often used in a more affectionate way, particularly for someone with shared experiences or beliefs.
Informal & Affectionate:
* Sweetheart: Can be used for a close friend, particularly if there is a strong bond of affection.
* Dear: A general term of endearment that can be used for friends.
* Good fellow: Indicates someone who is enjoyable to be around and trustworthy.
More nuanced:
* Bosom friend: A very close and trusted friend, often someone you share your deepest secrets with.
* Confident: A person you trust with secrets and confidences.
* Familiar: A person you are well-acquainted with and feel comfortable around.
Using phrases:
* "My dear friend"
* "A true friend"
* "A faithful companion"
* "A kindred spirit"
Ultimately, the best word or phrase to use will depend on the specific context and the relationship you are trying to describe.