Understanding your villain.
Yes, this story belongs to your main character, but what are they without a supporting cast?
The antagonist has goals too, and we need to understand why.
Ask the same questions you would ask of your main character.
What is their relationship with their parents? Were they adopted? Raised by mountain lions?
Did they have friends or were they a loner? Was this by choice?
Describe the most unforgettable moment in their childhood (good or bad).
How does the villain cope? Where did they learn these coping methods?
Do they trust people?
What makes them cry?