In a lot of Harry Potter's arse fanfiction, Dumbledore might make an appearance. He is such an important figure in the wizarding world, and his wisdom could be used to explain or deal with whatever strange situation is going on with Harry's arse. Snape could also be there, perhaps begrudgingly helping Harry when something related to his arse endangers the whole school. And of course, there would be the various Hogwarts students who might be curious or involved in the events surrounding Harry's arse.
Some fanfictions might explore a plot where Harry Potter's arse has a connection to another dimension. Through this connection, strange and wonderful things start to happen. For instance, items from the other dimension start to appear around Harry's arse. It could also be a portal of some sort, and Harry and his friends have to figure out how to close it before dangerous creatures from the other side come through.
Maybe it's an old - fashioned, floppy - brimmed hat that gives him a bit of a quirky look. It might be made of some rare magical fabric that has the ability to change patterns depending on his mood or the situation he's in. For instance, if he's in danger, the hat could turn into a more intimidating, spiky - looking pattern.
Hermione would definitely be one of his top allies. Her intelligence and knowledge of magic and history would be invaluable in helping Harry rule. She could advise him on laws and policies, especially those related to education and the treatment of non - human magical beings. Ron would also be an ally. His loyalty and his understanding of the wizarding world's less - sophisticated side could help Harry connect with the common wizards.
Hermione would be a likely colleague. She's highly intelligent and could be teaching a subject like Ancient Runes or Arithmancy. Ron could also be there, perhaps as a flying instructor or teaching something related to magical creatures. Neville might be a Herbology professor, given his love and talent for that subject. Luna could be teaching something really unique, like the study of magical creatures that are often overlooked or misunderstood.
Hermione would definitely be one of his allies. She is smart and resourceful and would be able to help Harry navigate the new political and magical situations he faces as a prince. For example, she could research ancient laws and prophecies related to his new status.
Voldemort and his Death Eaters would still be the main enemies. But now, Harry's guns would pose a new threat to them. They might not be used to dealing with such Muggle - like weapons, even if they are magically enhanced.
In some fanfictions, it could be an original character created by the author. Maybe a witch who was the result of an affair that his father had before he married his mother. However, this is completely against the canon of the original books, but in the world of fanfiction, anything is possible.
In some fanfictions, it could be Sirius Black. Some fans like to imagine that Sirius had a deeper connection to James Potter's family than just being best friends. They might create a scenario where there was some sort of magic or prophecy involved that made Sirius more than just Harry's godfather, but his actual father in this alternate story line.
Perhaps Hagrid could be his mentor. Hagrid has a lot of knowledge about magical creatures and their parts, which could be useful for Harry in his blacksmithing. He could show Harry how to use things like dragon scales or unicorn hairs in his creations.