A friendly and funny down-to-earth "best friend" who always has your back is a trope. It makes for enjoyable characters like Garrus, Chewbacca, Alistair or Edér.
I don't feel Alistair quite fits with the others there though. Isn't he more "the whiner" than "the best friend"? You know, the mandatory Bioware companion that spends most of his time crying and whining; Carth, Atton, Carver, Kaidan... Though I suppose he can be put in more than one box.
