Dorcas, Barty and Regulus
In third year, Dorcas was fascinated by aliens. She fully believed the various U.F.O. sightings and recordings, and went down conspiracy theory rabbit holes about abductions and the like. Regulus, Evan and Barty all teased her mercilessly for it (not Pandora, of course). Barty was especially torturous, mocking her any chance he got, but she stood her ground.
She was complaining about Barty to her friend, Emma Vanity, when Emma suggested that she get him back.
Dorcas cornered Barty in the halls by himself one day, and told him that she had conclusive proof that the Earth was flat. He was obviously skeptical, but she bombarded him with -mostly false- information, and he slowly came around to the idea. Three days had gone by, and Barty was fully sold on a flat Earth. At dinner, he excitedly shares all his newfound knowledge to his dormmates who are less than impressed. Dorcas is struggling to hide her laughter.
A couple of weeks went by and Barty started doing his own research on flat Earth, still fully convinced when Dorcas decided to up the ante. She researched other conspiracy theories to share with her friend. She told him about the lizard people that run the Ministry (which isn't that far fetched considering some of the things that happen in that building), that Quidditch bludgers are actually sentient creatures, and that the story of the Three Brothers is real. She told him about the Chamber of Secrets, that there is magic so dark it splits your soul in two and that magic comes from your vegetables and you have to eat them to become a powerful wizard (admittedly, that one was just a white lie that her parents told her as a child).
Barty became a full blown conspiracy theorist, and started doing extensive research on the topics. He left his notes all over the dorm, much to Evan and Regulus' irritation. He fixated obsessively over each new idea, and shared all of his discoveries with his group. No one took him seriously, except Pandora, who listened to him talk for hours, paid attention and asked intelligent questions, much to his delight. His theories became something of a joke inside their group, and they were all glad when he finally seemed to run out of steam about the topic.
What no one was expecting was for Regulus to ask for some of Barty's old research, years later, when their friendship group was splitting at the seems. Regulus was the most cynical and logical of their group, so it was laughable that he too was falling prey to the conspiracy theories. Indeed, he was teased mercilessly by everyone except Pandora, who once again, listened patiently to his theorising. Dorcas had started pulling away from the boys now, angry at their willingness to fight with the Dark Lord in the upcoming war, and tensions were high amongst them. When Regulus came to her, asking for her help to research this dark magic with him, Horcruxes, he called it, she refused, telling him it was just a stupid prank from years ago. She brushed him off, furious that instead of trying to mend their friendship and make the right choices, he was focusing on a stupid rumour that she had found in an old library book.
When it was announced that Regulus Black had gone missing and was assumed dead, Dorcas couldn't help but remember how she had turned Regulus down in the last request he had ever made her, and wondered if it would have made a difference.