Pacifica Northwest
A popular, rich, annoying girl who thinks she's the best, but she's just a fake version of the main characters, and she ends up being friends with Dipper because he's too dumb to know she's fake.
"She's rich, she's popular, and she thinks she's the best, but she's just a fake version of the real characters," said the ghost in the mansion.
"Pacifica is like a rich kid who thinks she's the main character, but she's just a sidekick who got lucky," Mabel said.
"She was the enemy, now she's Dipper's girlfriend, and she still thinks she's the best," said the llama in the zodiac wheel.