House: Gryffindor, 6th Year
Twin brothers Fred and George seem pretty much impossible to tell apart, so we won't try. Their main character trait is that they're both practical jokers. They invent all kinds of slightly disturbing, cool stuff: toffees that make your tongue grow several feet long and custard creams that turn you into a canary for a little while, and so on. They're starting to put together order forms for these things, which they call Weasleys' Wizard Wheezes.
They want to start a joke shop someday, but they need a bunch of money to do so. It's the lure of the thousand Galleons in prize money that leads them to try and enter their names in the Goblet of Fire even though they're too young to participate in the Triwizard Tournament. They wind up with long grey beards for their trouble, but they're good-humored enough to laugh about it.
Mrs. Weasley doesn't approve of their joke shop plans at all, which causes real tension between them. Much of Harry's stay at the Burrow during the summer is spent avoiding the fights that keep breaking out between mother and twin sons. However, Mr. Weasley doesn't seem quite as intent about preventing them from following their dream. When he sees Fred and George betting their life savings on Ireland winning the Quidditch World Cup but Bulgaria getting the Snitch, his only response is, "Don't tell your mother you've been gambling" (9.1).
Poor Fred and George then spend most of Goblet of Fire struggling to get their winnings from Ludo Bagman. He strings them along and strings them along. Fred and George write many letters, try cornering him in person when he arrives to judge the tasks, and even consider blackmail. They're that desperate for this money to get their joke shop off the ground.
This small plot arc gets its resolution in a completely unexpected way: Harry receives the prize money for the Triwizard Tournament…but he absolutely doesn't want to keep it. He hands over all of that gold to Fred and George because he's got a feeling that "we're going to need [laughs] more than usual before long" (37.163). So Harry becomes Fred and George's secret investor.