Hunting Trip
They tell him all the time that his father was a great man before The Incident.
"A really top-notch Auror," Gran says. "One of the greatest dark-wizard hunters Britain has ever seen."
They tell him other things, too, like that his father had been loving and brave and protective - but the word that catches in Neville's young brain is hunter.
"Dad's on a hunting trip," he whispers to himself on the nights when the world seems especially cold, "and he hasn't been home in a few days. But he's coming soon."
(Even after he's too old to think this way, there are moments when he needs so badly to believe in something that he can almost convince himself it's true.)
He carries on this way through the years of bullying, and he prays to nothing that one day he'll grow out of his chubbiness and grow into his teeth and grow up already. He stands up for himself, the way they all say his father would have wanted him to - and he hates the way they talk about him as if he's dead, because he isn't dead, he's just gone, he went out on a hunting trip and he hasn't been home in a few days or weeks or months or years or decades but he's coming.
He marries a nice girl. (Dad isn't coming back, is he.) Finds a good job. (He can't come back.) Builds a decent life. (The damage is permanent, they've always known that.) People tell him his father would be proud.
(There is nothing to do but carry on.)
It isn't until he becomes a father himself that he realizes he never knew his dad at all, and that the hero on the hunting trip is only a figment of his imagination.
[One of Every Letter: H]
[Herbology Class: Carry On Wayward Son by Kansas (Bonus points if you use the dialogue "Dad's on a hunting trip, and he hasn't been home in a few days")]