Avatar: the Last Airbender x HP
Fem!Harry
Reincarnated!Harry
Set 100ish years before the series, before Aang froze himself in the iceberg.
Harry (water), Aang (air), Bumi (earth) & Kai (fire) were the best of friends.
They had a group portrait done when Aang was 12 (slightly before iceberg), where they were all wearing their most formal clothing. They each got a copy. Harry as the only girl sat in a chair in the middle while the boys stood around her.
I kind of imagine, that Bumi was always in line for the throne in Omashu, and with the Air nomads knowing that Aang was the avatar since almost birth, he would be like royalty/nobility. This makes me want to make Harry a Nobel/Royal, and Kai as well (like in his case a high Nobel [kind of like Mai/May]). Like maybe these two are the heirs to a smaller nation/island in their respective kingdoms/countries. Like how Bumi is a king but not the Earth King.
And like they all meet at like a national meeting/conference for rulers or something, when they are like little kids and are just thrown together and become the best of friends. Who decide to visit each other all the time (they tell their parents/guardians that it is to enhance their worldly knowledge and people skills).
Also Harry & Kai live just as long as Bumi does, the trio are holding out so that they can see their friend Aang once more (they all suspect/know that he isn't dead, just missing). And these three would be like the founders for the White Lotus or something. And they still continue to visit each other and keep in contact even with the Fire Nation attacking/conquering everything.
Maybe Harry is the princess to the Swamp People, so her clothing has more plants incorporated, and more blue greens. (The Foggy Swamp Tribe, separated from the South Pole and established themselves in the earth kingdom). I think I would change their culture slightly to make them seem more important and established (since they don't have a canon representative for the United Republic Council. Though there clothing will remain more or less made from plants and bark [no shoes!], though their really formal clothing for when they go to these international "meetings" could be more attuned to what other water nation people wear, especially their 'royalty'). Also the Foggy swamp people tend to be paler skin, with dark hair and tend to have green eyes instead of the blue of other water tribes (this actually fits Harry really well!)
If the three of them join the White Lotus they would be Grand Masters or Grand Lotus level, since they would have been there for a long time.
Maybe Kai is a part of the Sun Warriors? I know they like the Swamp people were more or less unknown (like the other tribes didn't know they still existed), but it would fit in with the theme of Aang's friend being from 'unknown' or 'little' kingdoms. Though to keep the 'secret' or his people Kai could/ would just claim he comes from a small offshoot of the fire nation that was around since before the forming of the Fire Nation and it's people. Though he could end up spilling the beans to his best friends at some point, maybe after Aang disappears?
And I have no idea how Bumi became king since there is no known kings/rulers of Omashu listed before him in the wiki, well there was the 'founder' Oma.