This is a bit old by this point, but still extremely awesome. Arc Matrix is a post by Robin Laws. At the time of that writing, he was getting up to speed with D&D 4e and looking for ways to ensure that characterization wasn’t lost in the shuffle of a new system.

His solution is a matrix of long-term influences that characters can have on one another, with two major outcomes. It’s pretty neat, and I suggest you read it. I’m tempted to do something similar when I get my act together and start running a long-term game.

While this was in the context of D&D, you really could use it anywhere. I could definitely see this being useful in a Vampire: the Requiem game, for example.