Surprisingly, We Have To Start With The Hero

In order to know our villain, we have to understand our hero.

Write down five positive characteristics of your hero. For i.e

Now, write the exact opposite of those characteristics next to it, along with behaviours of the opposite. If you've already got a novel/short story on the go, use pieces from your work.

NOW

Do the same with your villain. 5 aspects that make them a villain, the opposite of that characteristic and behaviors that reflect. Compare the two, how well do they match up? DON'T CHEAT.

Alright, I'm sure you've already guessed: The opposites are what your villain should display. HOWEVER, keep in mind your villain is NOT the polar opposite. In fact, your villain should be your characters SHADOW. As close as a shadows lingers when you walk with the sun to your back, is how close your villain should relate to your hero. 

Your villain should also be as dark as a shadow is. Most of the time, we forget about our shadow, do we not? We're so used to having a shadow that it goes unnoticed and seems irrelevant to whatever we're doing. In the same way, your villain should be a dark follower. There, but a ghost.