Creating Villains
Villains are a particular type of Non-Player Character (NPC) whose goals and actions put them in direct conflict with the Player Characters (PCs). Villains differ from your everyday enemy in that they are specifically working against the actions of the PCs, or visa versa. Not every story needs to have a Villain, but Villain's provide the PCs direct opposition and someone to dislike or even hate. This gives them motive and drive, spurring them on to stop the Villain before his nefarious plans can be realized.
Developing Personality and Background
Just like with all NPCs, you want to create a distinct personality for your Villain (see Creating NPCs for details). You want your Villain to be memorable, so make sure that he stands out from the crowd. Try to come up with a distinct voice, gesture, or mannerism that the PCs can tie to the Villain, something that the players will pick up on without you having to say the Villain's name. Also consider how your Villain's personality will affect their interactions. How will they responds to threats? To challenges?
Your Villain's background is likely more important than any other NPC. It helps you to determine what their motives are and why they do what they do. The more you understand the "Why", the better you will be able to determine how they act and respond to the PC's actions. Your Villain's background can also help you to form goals for them, which are used to drive your plot and give purpose to your adventure.
Villains and Plotlines
Since Villains have their own motives and goals, a good Villain concept can be the starting point of your adventure. By designing a Villain and understanding his wants and desires you can easily set him up in a position to oppose your PCs. By knowing what your Villain will do to achieve his goals, you'll know how he will respond as the PCs interfere. Your Villain's actions and the PC's reactions will drive the story and advance the plot for you. Having a good Villain with distinct goals and motives can be all you need to get your plot started out.
Defining your Villain's day to day activities and understanding the timeline of their plans also helps to keep your plot moving. Remember the Villain's don't remain idle just because the PCs decide to take a three day shopping trip to a nearby town. Let your Villains timeline dictate the pacing of the adventure. As the PCs see the results of the Villains actions, you can create a sense of urgency that keeps your story moving.
Creating Villain Statistics
At some point during your adventure, the PCs will likely come into direct contact with your Villain. Sometimes this will be in the form of a Social Encounter, other times Combat may ensue. To give your Villain versatility, consider working them up as a full Character (as if you were making a Player Character), with a complete spectrum of powers, feats, and skills (see Creating Characters for instructions). While this can be time consuming and more difficult to run, it allows you to have the full versatility of a Player Character and provides the PCs with a greater challenge. Most Villains don't work alone, however, so combats with them will likely include some other henchmen (usually Simple Monsters) to assist him. Alternatively, you can use a Complex or Legendary monster entry to represent you Villain. This method is less time consuming than creating a unique Villainous character, but doesn't offer the same kinds of versatility.