Well, no matter which kind of best friend you have but you should appreciate all of your best friends. At least they have tried their best to be your best
But the topic itself won't be too deep. You won't tell the whole story but you'll conceal some parts because you still can't really trust them. This can be called as Friends for Gossip (FFG).
Third, they who know you well enough because you spent a lot of time with them. Usually you meet them because you guys are in the same class or same workplace, so it enables you to meet oftenly.