The Best Defense and its sequels are an epic Harry Potter/Yu Yu Hakusho crossover, AU for 5th-7th years in the HP universe.

Hiei scowled, obviously not content with the situation and probably feeling trapped, since the malfunctioning gate had vanished behind them, leaving them to find another way back to their own territory. The little youkai began scouting the area, no doubt seeking an exit. Kurama started to prowl as well, but he was more interested in discovering what this temple had to offer. He wouldn't be averse to taking a few souvenirs with him.

...with Ogata Megumi's voice who constantly wear pink and occasionally transform into ancient, magic-wielding nonhumans with long silver hair and auxiliary appendages and who have powerful, dark-haired best friends...

Tag for the Best Defense series - all the side fics

"So. You're that one kid fightin' some one-guy-against-the-world war, with space tech and a megalomanic with way too many corrupt rich guys on their side, and slightly better space tech on your side." Heero looked up from the cot he was loosely chained to, giving a stony glare to his captor. The half-dressed, tattooed teenager, with enough hair to rival 02 and a notable lack of weapons hidden in his torn jeans, didn't look impressed. "And they walloped you with somethin' freaky that tore through some dimensions, which is why your better space tech is currently making like ugly modern lawn art in what's left of my courtyard."

Kurama shrugged. Technically speaking, he'd had full functionality since he was ten, enough youki permeating the cells to allow him to use said youki as well as he had as a youko. And with Suzuki's potion boosting the process, he'd been able to shift to his half-human form several years early. Now, though, his youki had infiltrated right down to the DNA, and he should be able to shift to a full fox. Finally.