So now that we’ve gotten past the “not your mate” thing… Here are the qualities that make for a best friend, IMHO:
1. someone you can talk to about anything and everything, knowing any advice you get will be from the perspective of looking out for your best interest.
2. knows you better than anyone else does.
3. shares your same goals and perspectives on life.
4. never get tired of “hanging out” with him or her
5. even when you disagree, you can never stay mad at each other.
6. things seem amiss if you don’t talk to each other a few times a day.
There’s probably more, but that’s a good start.