1. Make Them a Worthy Rival
*same level
*or better than the main character
2. Give Them a Believable Motivation
*reasonable behind the evilness/badness
*they believe passionately in what they are doing
3. Your Villain is Still Human (unless otherwise.)
*know their backstory
*make them a believable person
4. If They Have a Suprise Reveal, Give Them Henchmen. (minions)
5. Validate seriousness