The Hex Maniac is a Trainer class I definitely remember seeing in the Kalos and Hoenn games. They are spooky girls in black dresses or other dark clothing that use Psychic-type and Ghost-type Pokemon. While there are Trainer classes in Pixelmon that prefer Pokemon of particular types (Bug Catchers and Bug Maniacs like Bug-types, Fisherman and Swimmers like Water-types, Firebreathers like Fire-types, Gardeners like Grass-types, Miners like Rock-types, Blacksmiths like Steel-types, and Sports players tend to like Fighting-types), there aren't any existing Trainer classes in Pixelmon that particularly like Ghost-types or Psychic-types like the Hex Maniac does.
The Hex Maniac will primarily use Ghost-type Pokemon and Psychic-type Pokemon. However, they may have some other Pokemon sprinkled in related to the theme such as ones that are related to magic and curses or could learn Hex.
Here's a sample set of Pokemon for this Trainer class:
- List of Possible Pokemon | Show
As for biome categories, I feel the Roofed, Magical, and Evil categories would be great fits. Additionally, the Taiga, Swamps, or Mushroom categories may also work.
Alternatively, Channeler, a Trainer class from the Kanto games that uses Ghost-type Pokemon, could be used instead. Though they have less Pokemon associated with them and only used the Gastly line in the main games. I feel the Hex Maniac class should be prioritized because is more interesting, already has an NPC skin, and uses more kinds of Pokemon.