Sometimes, you get characters who are, for lack of a better term, inseparable. Not in the physical sense, mind. They don’t always need to be in the same place at the same time—yes, they can be joined at the hip, but where’s the fun in that? No, this sort of inseparability is a result of mutually dependent backstories; they have been sufficiently integral to each other’s lives, histories and character development that you simply cannot have one without the other. To take them completely separately—to completely erase one from the other’s history, thoughts and influences—would require the creation of an alternate universe. (As a bonus, if you’re the kind of person who does alternate universes, you’ve found that your existing ‘verses tend not to split them up.) In short, these characters are a demonstration of mutually dependent characterization.
As an element of writing and characterization, I consider this to be a pretty awesome concept. For one thing, it shows that the characters are neither The Only Thing Worth Considering nor walking cardboard. A character who can become part of a mutually dependent characterization dyad isn’t static, but capable of change—a character who can influence another into a mutually dependent characterization dyad has presence and an impact on the world about her. Both doing both, then, is a good sign.
For another, it gives a chance to see other facets of the same character. While it isn’t always the case, most characters who display mutually dependent characterization will tend to act at least somewhat differently with the other character in the dyad than with other people. It won’t necessarily be more open (see all the different dynamics below—the relationships just don’t always allow it), but it certainly tends to be interesting, whether it’s a vulnerable side, a temper otherwise kept completely in check, the results of a bit of otherwise confidential information….
One of my favorite parts, though, is that the kind of character relationship doesn’t prevent mutually dependent characterization from happening. Sure, we’re most used to lovers and archenemies, to rivals and relatives (particularly close relatives), but you can get it just as well from mentor and student, from superior and subordinate, from coworkers or close friends… you get the idea. And with this amount of difference comes an even greater variation in the effects of each character in a dyad on the other.
Last is the fact that this is a natural result of two characters having to deal with each other in whatever way, shape or form for prolonged periods of time. If their characterizations weren’t unique to the dynamics and circumstances they found themselves in, that would be a sign of a writing problem.
This isn’t to say that mutually dependent characterization is without risk—but risks or none, the rewards are more than worth the effort.