Leader of the Quack
Harold is first seen on his horse in his knight armor outfit welcoming Donald, Daisy, and Huey, Dewey and Louie to their kingdom of Quaintinia. He is about to attack Kent Powers, whom he mistook for a giant (actually a hot air ballon resembling Kent) then an intruder arriving in Quaintinia, but Daisy stops him. He then takes Donald Duck to his castle to make him their king.
When Daisy asks Harold how Donald became king and Donald remains oblivious as well, he tells the others that Donald once saved their land from a dragon named Nelly during his last visit to their kingdom. What really happened was that when Donald was about to take a picture, some knights on horses ran into him and he angrily decided to leave, unaware that the land was being attacked by Nelly, and while he backed up his car, he accidentally hit a catapult which launched a large rock that hit a wooden gate, causing water to flow over Nelly and make her lose her fire. Harold believed that Donald saved their kingdom and therefore, proclaimed him their king since the law states that whoever defeats the dragon, is hereby crowned king. Sadly, since Donald left that day, the vile Prince Gaylord has ruled the land from his tower. Back to the present, Harold makes Donald go to sleep after promising to serve and protect him from Gaylord's attempts to slay him and then the next day, he gives Donald food, which gets eaten by another person (the food taster) instead, and then he makes Donald boss people around.
When Harold reads a scroll that says everything Donald says when he bosses people around, Huey, Dewey and Louie show up. Harold then takes Donald and his nephews to watch a battle in their Medieval Tournament games.
Harold is last seen when the residents of the kingdom believe that King Donald's new rules are ridiculous and absurd and that Dewey is the "leader" of their rebellion but all of this is interrupted when Nelly arrives with her fire returned, thanks to Gaylord and hot sweets bought from Donald's nephews and Harold states that the law says that only the king can defeat the dragon.
It is possible to conclude that after Donald gave Nelly the crown since Donald was fed up with being the king plus she loved the outfit the same way she loves gifts given to her by others (making her a "sucker for gifts" and which explains her alliance with Gaylord) and Gaylord was defeated by Nelly, Harold has now sworn to serve and protect Nelly as the new queen of Quaintinia.