Original characters. Essential for the author that wishes to add a bit of spice to their fanfic.
I'm not sure how one comes up with OCs. Perhaps it comes to them in a flash, or over the course of years?
Common knowledge though is that the balance of power is essential when creating an OC. Go overboard, and the OC ends up Mary Sue, which if anyone doesn't know the term, is plain overpowered, no weaknesses whatsoever, and pretty much flipping off everyone in the universe the character is in. No fun at all.
I think my favorite OC in dA which I know of so far is Seth, which belongs to sethb1 of the Touhou Walfas community (I hope I got the nam