Grunkle Stan and Ford are likely to be in the fanfiction. They might be the ones to initially discover Dipper's powers or help him understand and control them. They have a lot of knowledge about the strange happenings in Gravity Falls, so they are important characters. Also, Bill Cipher might make an appearance, either as an enemy trying to take Dipper's powers or as a source of information about the powers in a more complex and devious way.
Obviously, Dipper is the main character as the title implies. His sister Mabel is often included too. She might react in various ways to Dipper getting powers, like being excited or a bit jealous. Wendy is also a common character. She could be a source of support or a love interest for Dipper in some fanfictions.
The main characters interacting with Dipper would include Mabel, who would be very curious about what Dipper can do with the powers. She'd constantly be asking him to show off new tricks. Ford would be the one trying to keep Dipper in check and make sure he doesn't go down a dark path with the powers. Then there's Stan. Stan would be a bit skeptical about Dipper having these powers at first. He might think Dipper is getting in over his head. But as time goes on, he'd start to see the potential good that Dipper could do with the powers, like protecting the Mystery Shack from supernatural threats. And of course, the Manotaurs might show up, seeing Dipper as a powerful being and either challenging him or trying to befriend him.
Another plot might involve Dipper having to keep his powers a secret from his family and friends at first. He struggles with this as he tries to figure out how to control them without raising suspicion. For example, he accidentally makes things levitate when he's around Mabel, and has to quickly cover it up.
Well, Dipper might start causing chaos just like Bill. He could use his new powers to play pranks on Mabel, like turning her sweaters into strange, living creatures. Or he could use the power of mind - reading to know what everyone in Gravity Falls is thinking, which would lead to some really awkward and funny situations.
Well, a frequent theme is the change in Dipper's personality. When possessed, he could become more aggressive or withdrawn, which creates a contrast to his usual curious and determined self. There may be a theme of redemption too. If Dipper does something bad while possessed, the story could be about him making amends once he's free. And sometimes, the fanfiction might use the possession as a way to introduce new powers or abilities that Dipper has to learn to control.
One common theme is self - discovery. When Anna gets powers, she often has to figure out what she can do with them and how they fit into her identity.
Well, perhaps he'd start to have a more chaotic view of the world. He might be tempted to use the powers for his own gain at first, like getting rid of his daily chores or getting better grades easily. But then he'd realize the great responsibility that comes with such powers and struggle to keep them in check.
Often, there might be a villain. This could be someone who wants to use the power of the plants for evil, like a dark sorcerer who wants to drain the life from the plants to gain more power for themselves. And of course, there would be friends of the OC, maybe other people with different powers who team up with the OC to protect the plant - powered world.
Hermione is a very common character. Given her intelligence and magical skills, she often plays a crucial role in saving Harry. Ron is also often involved as Harry's loyal friend. Dumbledore, even in fanfiction, is a character who might show up to save Harry or at least guide the others in the rescue.
Well, of course, Caillou himself is the central character. His parents also play important roles as they are the ones who ground him and teach him lessons.
In many of these fanfictions, there could be a rival trainer. This rival might be jealous of the captor for getting Darkrai and try to steal it. Also, there could be a Pokémon professor - type character who gives advice on how to handle Darkrai. For example, they might know some special techniques to keep Darkrai calm while captured.