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.
One possible plot could be that Harry discovers he has Haki during his time at Hogwarts. He first realizes it when facing a Dementor and his Haki flares up, scaring the Dementor away. Then, he starts to secretly train his Haki with the help of Hagrid, who has some knowledge of strange powers. As he gets better at it, he uses it to sense the true intentions of people around him, like when he suspects Snape. Eventually, he uses his Haki in the final battle against Voldemort, giving him an unexpected edge in the fight.
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.