A true, quality, best friend is someone you know you could say just about anything to and they trust and understand you and vice versa. Communication is reciprocative and you try to consider each others' feelings, time, and energy, but you also try to be independent enough and not depend on them in a way that infringes on their psyche and feelings. Sometimes, you try to help them in some way back too.
Initially, communication should be reciprocative and caring enough. Even if you don't meet often, explanations are enough even if brief, and your time and energy is not infringed on or this person initiates on his/her own to make up for his/her mistake or mishap. Such as, if they left you hanging on meeting up, they offer to buy you a meal/drink and meet you in-person at your convenience the next time around- something like that.
I have one, not sure if I still have him as a bestie, but we're still at least friends. He has too much BPD and Depression to keep contact and hang out, but we keep contact still. One time, we spoke for 6 hours at length- him for 3 hours and me for 3 hours. We helped each other sort through each others socially related situations and talked about how best to move on without infringing on anyone else and moving on with our own lives. Not many people at that time could give me that needed time in-person nor advise me in just the right way. Even if a few others could, could I trust them like I could this person?
Part of having best friends is building trust and reciprocative communication. You can't hide away and just expect someone to "be" your friend. You have to put yourself out there and take a risk. It can be scary for some.