Who's your dearest friend who's been and gone through thick and thin with you and how long have you known them for?

Who's your best friend? What's the story of that friendship? That can be a friendship that ended or is still going on. I'm interested in hearing the kind of 'best friend' people have, a close connection they had in friendship. The gender doesn't matter, but it shouldn't be your SO. Who knows you better, your best friend(s) or your significant other?
If you consciously reveal more about yourself to one over the other, is there a reason? When was the moment you realized that your best friend wasn't your best friend? Why is your best friend your best friend? How did a friend of yours become your best friend?