Maybe an ancient sage in Naruto's own world who has long - been in seclusion. This sage has knowledge of different power systems beyond just chakra. He notices Naruto's pure heart and strong will and decides to pass on his knowledge of haki to Naruto, believing that Naruto can use it for the greater good of the ninja world.
In 'naruto haki fanfic', Haki can change the Naruto universe by adding a new layer of power. For example, it can make battles more complex as characters now have to consider both chakra and Haki. It might also change the way characters interact with each other, as those with Haki could sense things differently.
Percy often uses Observation Haki to anticipate the attacks of his enemies. He can sense their movements and intentions, which gives him an edge in combat, especially against fast - moving or sneaky foes like some of the monsters in the Percy Jackson world.
Haki can completely shift the power dynamics. If a character like Ron suddenly discovers Haki, he could become a much more formidable opponent. He might be able to use it to intimidate Death Eaters or other dark wizards, adding an edge to his character that wasn't there before.
In some fanfictions, Naruto might discover haki during a particularly intense battle. Maybe when facing an enemy with an unusual power that his regular jutsu can't handle, he unknowingly taps into his latent haki ability.
An OC's Haki can be developed through intense training. Just like how Luffy and the others train in the One Piece world. They could face difficult opponents and learn from each battle, gradually strengthening their Haki. For example, if they constantly fight against Logia - type devil fruit users, their Armament Haki would improve to counter their intangible forms.
I think Conqueror's Haki would suit Naruto. He has that natural leadership and strong will, just like characters in One Piece who possess Conqueror's Haki. It would be a great addition to his existing confident and determined personality.
Dumbledore could be the one to teach Harry about Haki. Dumbledore, with his vast knowledge of magic and the unknown, might have come across Haki in his travels. He could recognize the signs of Haki in Harry and take it upon himself to guide Harry in understanding and controlling this new power. He would use his calm and wise approach, just like when he teaches Harry about other aspects of magic.
Yes, in many Naruto fanfictions, authors often give Naruto the power of Haki. It's a creative way to blend the concepts from One Piece (where Haki exists) with the Naruto universe. This can lead to really interesting storylines, like Naruto using Haki to sense enemies' intentions better during battles or using it to enhance his existing jutsu.