How one character refers to another can indicate the relationship betwee the two.
Pecking order can also be established:
There are lots of terms of endearment to show a special relationship between two people. This website lists about 200 of them!
Nicknames might also be appropriate. for example, anyone with the surname Smith might be called Smudger. Indeed, the Forces are renowned for assigning nicknames to new recruits. Check out lots of examples here.
Wikipedia lists nicknames applied to famous people, places and things here.