Holly Graham is the true main antagonist of Scooby-Doo! Night of 100 Frights.
Role in the game
She dressed up like a phantom called the Mastermind and made holograms of classic Scooby-Doo monsters because She wanted to frame her uncle, who is an inventor, so that she could steal his inventions. She used holograms of herself to give herself an alibi.
Her name sounds similar to the word "Hologram" and Shaggy makes a joke about this at the end of the game.