I dont exactly have many friends offline... after all, keeping to myself as much as I do I dont exactly meet people much... but what few I do know have proven themselves in a whole pile of ways.
But the things that really stand out for me is that they deal with my general weirdness with understanding and acceptance. They know I'm autistic and I'm like 99.99% sure my closest friend has either completely figured out or at least heavily suspects my gender issues, and when it comes to ALL of that, they just roll with it, no problems. Always have. Even when I'm in full cosplay mode (AKA "girl mode") at conventions and jacked up on caffeine and sleep deprivation (I get really loopy... my own fault, really, nobody else to blame on that one. I did eventually learn to tone that problem down).
And they also are good at tolerating my need for distance. I aint always so good at communication, sometimes I can go a good while without talking to any given person, and that's definitely an ongoing problem for me, but... they get it. They always have. And I'm thankful for that. Same with family too, they know I can be really distant but they just allow that as it keeps me from getting overwhelmed. Though I am trying to change it a little as I know it bothers my mom in particular. But in any case, yeah, friends have always been fine about that.
Overall... they're fine with who I *actually* am, instead of the messed up, fully masked version that I show everyone else.
Also, banter. You KNOW you're close friends when you can trade verbal barbs during some sort of competitive activity and then you both end up on the floor trying not to pass out from laughing too hard.
I do rather wish I could do more for them though. Used to be, what I'd do for my closest friend typically involved my car. He couldnt drive, for the longest time, didnt get his license. So, he needed to go somewhere? Just say the word, and boom, I'm headed over there. Took him to work a lot, actually. No problem to me, I enjoy driving and we could chat about random things the whole way.
Though in recent times, after like 20 bloody years he got a license and a car, so that's not needed anymore... I dont know what else I can really do, that was like the extent of my usefulness.