I've had two, of a certain kind.
The first one arrived in the traditional form, 12-year-old girl, in my class. She knew everything about me. At 14 I told her my final, biggest secret, and I said that I felt like my soul was naked. Then Carolyn moved away, thousands of miles. In the beginning, we exchanged fevered letters, endlessly proclaiming out status as each others' best friend, ever, but eventually the letters thinned. Now I can't find her.
The second one surprised me. I was an adult, married, and used to living without a best friend, without that kind of best friend anyway. Then I met a man older than my parents to whom I could tell all my secrets.
[yes, this is horrible. But I'm doing something...anything]