Ernie Macmillan is a Hufflepuff second year, a friend of Justin Finch-Fletchley's, and a pureblood who can "trace [his] family back through nine generations of witches and warlocks" (11.151). Ernie doesn't tell Harry about his pureblood status because he hates Muggle-borns or anything; he is just afraid that Harry will attack him if he doesn't assure Harry he's a pureblood. Ernie is the Hufflepuff primarily responsible for spreading rumors that Harry deliberately targeted Justin Finch-Fletchley for Petrification because he has Muggle blood. He is also the one who says outright, "That's probably why You-Know-Who wanted to kill him in the first place. Didn't want another Dark Lord competing with him." (11.141).
Yet, even if Ernie is a rumor-monger who scares easy, he's a basically decent and honorable guy. After Hermione is Petrified, Ernie approaches Harry in Herbology to say, "I'm sorry I ever suspected you. I know you'd never attack Hermione Granger, and I apologize for all the stuff I said." (15.19). Ernie shakes Harry's hand. Ernie is a bit of a prig and a know-it-all, but his heart mostly seems in the right place. Anyway, at least he apologizes for making Harry's life harder, which is more than most of Harry's enemies or former enemies do.