Often, the main characters are Harry and some of the Hogwarts professors. For example, Professor McGonagall could be involved. She might be the one who assigns Harry the task of polishing the broomstick in the first place, and she could be observing his progress and offering some strict but useful advice along the way.
One popular 'Harry Polish the Broomstick' fanfiction could be one where Harry discovers a magical way to make the broomstick not only super shiny but also gives it new abilities, like flying faster and being more responsive. It might involve him using some rare spells he learned from an old book in the Room of Requirement.
Harry Potter himself is the key character in 'Harry is burned' fanfictions. But other characters like the Weasley family as a whole can be involved. Mrs. Weasley might take on a motherly role towards Harry, trying to nurse him back to health. And Fred and George could use their magical inventions to try and ease Harry's pain or discomfort from the burns.
Often, you'll see Harry Potter himself as one of the main characters. Hermione and Ron might also make an appearance. From Lost Girl, Bo could be a main character. She has a strong personality and her Fae abilities can add a lot to the story when combined with the magic of Harry Potter. Dyson is another one from Lost Girl who could be involved. His loyalty and strength are traits that can play well in a fanfiction set in both universes.
Well, obviously the Marauders which include James Potter, Sirius Black, Remus Lupin and Peter Pettigrew. They are the central characters as they are the ones doing the watching. Harry Potter is also a key character since they are watching his life. Then there are often other characters from the Harry Potter series like Dumbledore, Hermione Granger, and Ron Weasley who pop up in the fanfictions as part of Harry's story that the Marauders are observing.
In many of these fanfictions, Harry is the central figure, but there are also original Hufflepuff characters created by the authors. These characters might be new students who have some special abilities or knowledge. For example, a young Hufflepuff girl who has an uncanny ability to communicate with magical creatures. Harry teams up with her and other Hufflepuff students to explore a part of the Forbidden Forest that has been sealed off for centuries. Their combined skills of Harry's bravery and the Hufflepuffs' unique talents make for an exciting adventure.
The main character is usually Harry Potter himself. Since it's about a neglected prodigy, Harry is at the center. He's often portrayed as having more power or potential than what was shown in the original books. Sometimes, Hermione and Ron also play important roles. They might be the ones who first notice Harry's true potential when others overlook it.
Since I don't know the exact 'little broomstick story', it could be a young girl or boy who is the owner of the little broomstick.
Obviously, Harry Potter himself is a common character. He is the main protagonist in the original series and often the central figure in fanfictions related to the broomstick as well. Hermione Granger and Ron Weasley also frequently appear, as they are Harry's closest friends and are usually involved in his adventures.
Harry Potter himself is often a great character in these fanfics. His natural talent on a broomstick can be explored further, like in a fanfic where he discovers new tricks on his Firebolt. Ron Weasley can also be interesting as he sometimes struggles with confidence on a broom, and a fanfic could show his growth in this area.
Harry Potter himself is of course the central character. He is the one with the blocked magic that needs to be removed.