Ernie Macmillan

Ernie Macmillan is a Hufflepuff student in Harry's year. He is described as a stout boy, and is descended from nine consecutive generations of pure-blood witches and wizards. His series debut is in Chamber of Secrets, in which Ernie believes that Harry is the Heir of Slytherin after he is witnessed speaking Parseltongue to a snake threatening Justin Finch-Fletchley, who is later petrified. However, after Hermione meets the same fate, Ernie realizes his mistake and apologises to Harry. In Goblet of Fire, he wears a "Support Cedric Diggory" badge and again gives Harry the cold shoulder after Harry enters the Triwizard Tournament, which Ernie believes is an apparent attempt at further glory, but they mend fences after the First Task.
Ernie becomes a Hufflepuff prefect in his fifth year, and joins the D.A. Ernie is among the few Hogwarts students who openly side with Harry's and Dumbledore's claims that Voldemort has returned. He deals with the pressure of O.W.L. exams by studying as much as eight to ten hours a day. His hard work and determination pay off, as he shows up in Slughorn's Advanced Potions class in his sixth year. During the climax of Half-Blood Prince, he emerges from the Hufflepuff common room as Snape, Draco and Harry run down a corridor during the Death Eaters' raid of Hogwarts; he attempts to ask Harry what was going on, but Harry blows by him with no response.
In Deathly Hallows, Ernie joins the D.A. in the Battle of Hogwarts. When McGonagall addresses the students in the Great Hall about Voldemort demanding Harry's surrender and the urgent need to evacuate, Ernie shouts out from the Hufflepuff table, "And what if we want to stay and fight?" He, Luna and Seamus conjure their Patronuses (Ernie's takes the shape of a boar) in their attempt to aid Harry in his battle against the Dementors.
Louis Doyle portrayed Ernie in Chamber of Secrets and Goblet of Fire, and voiced the character in the video game adaptation of Order of the Phoenix.