(1)Harry James Potter
(8)Peeves
(12)Augusta Longbottom
(4)Bill Weasley
Don't think I need to explain the first one. As for the second, the eldest Weasley child Bill is easily among the most prominent "background characters", actually appearing in quite a few scenes, fighting in quite a few battles, and sustaining quite a few injuries from them. Augusta, though a remarkable matriarch for sure, only gets the two scenes (that I recall)- one at the hospital for Magical Maladies and the other just before the Battle of Hogwarts, a boggart cameo, and a few Howler letters. I feel like Mrs. Longbottom may get more mentions than Bill, but by virtue of actual concrete screentime, Bill is the clear winner here. But that's not the sole reason why I'm voting him. He's just a really cool, well-rounded character. While I may not agree with his taste in women, I can definitely respect his courage (demonstrated at the Battle of the Astronomy Tower), his intelligence and ability to lead (Prefect and Head Boy), and his loyalty to his family at all times (take a page, Percy).
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Arcanine: The best pokemon ever.
Dewott: The cutest pokemon ever.