The Riddler or Keyser Soze?
After figuring out the villain’s motivations and likelihood of redemption, you probably already have an idea of the villain’s personality. Still, there are a few considerations remaining. For example, does the villain have a trademark, like The Riddler, or is the villain known for being untraceable, like Keyser Soze from the movie The Usual Suspects?
The villain with a trademark can be either the Nemesis or the Hidden Mastermind. Trademarks can be left intentionally (riddles, origami birds, a “Z”) or unintentionally (the smell of cigarette smoke, a particular weapon always used when killing, an odd-looking footprint). Intentional trademarks may indicate that the villain is confident, playing with the characters, and/or subconsciously wants to get caught. Unintentional trademarks may indicate that the villain is a little careless, unable to avoid leaving certain marks, and/or subconsciously wants to get caught. You decide.
The villain who is perfect and never leaves any clues behind works best as the Hidden Mastermind—after all, the whole point of the Nemesis is that s/he keeps showing up over and over! A Keyser Soze is a little tricky for the DM to handle, though, because the perfect Hidden Mastermind will never be Unmasked. The DM must assume that at some point or another this “perfect” villain makes a mistake or is betrayed. Put some thought into this one—the more interesting you can make the villain’s misjudgment, the better.
There are a few other personality traits to consider, too. Does the villain have a fatal flaw that the characters can exploit? (Overconfidence, underconfidence, weakness for the opposite sex, a hot temper, a hobby, a loved one?) Does the villain have any likable traits in an otherwise unlikable personality? (Never harms children, paints roses, composes beautiful music?) These and other traits will give the villain some depth and possibly provide hooks for the characters to build a plan around.
I believe that the best villains are the ones the characters end up liking or admiring despite themselves. If you can create an archvillain who touches the characters at some level, then whether or not they finally ending up killing the villain, you can be certain that your players will be talking about the adventure or campaign for years to come.