I'm warning you people, I'm going on assumption here so feel free to correct me and tell me I fail for ranting about something I'm not familiar with.
In my experience, though, if you've been best friends with somebody since FOREVER, you tend to get pet names hammered out early on. If you end up with any pet names at all. So Sora probably won't call Riku Ri-chan or 'Ku-chan or whatever and vice versa since he hasn't already. Even if they end up in a relationship, I don't So-chan's gonna stick. I figure any romantic relationship they have will be more friendly than OMGMUSHYSQUEEEEE despite the fact that they LUV each other, anyway.
Also, while Axel probably would have teasing pet names, Roxas and pet names just does not compute.
P.S.
This is Riku:
This is Ku-chan:
And guess what Ku-chan's real name is. Sora. Creepy