So, first of all. Lets just acknowledge that Matthew Lewis made the most stunning physical transformation of all the Harry Potter cast. And isn’t it just so perfect that his character, Neville Longbottom, made the most stunning personality transformation? From shy, stuttering, terrible student to confident, self possessed leader of Dumbledore’s Army. It’s incredible.
Neville is (like so many other characters, honestly) set up as a kind of foil for Harry. While Harry was raised Muggle, Neville was raised as a wizard. Neville is shy and self-effacing, Harry (despite the abuse he suffered) was snarky and confident. Harry was talented – both magically and on his broomstick – and didn’t seem to struggle at all in school, except against laziness and death. Neville struggled and strived for every success, because everything was difficult for him (though it has been pointed out that up till OotP Neville was using his father’s wand, and the wand chooses the wizard, so…). Neville could have been The Chosen One, but Voldemort chose Harry instead, leaving Neville and his parents to the Death Eaters.
I think that last piece is really the crux of the differences between Neville and Harry. Harry’s parents were dead, and Voldemort wanted him dead as well. Harry, as an individual, was a danger to Voldemort’s very life. Neville lost his parents, too, though in arguably a more tragic way, to the Death Eaters. He had a huge grudge against them – they didn’t seem to have any care for him (do we know where Neville was when his parents were captured and tortured?) but he had a huge grudge against them. So he ended up leading an army of students against them. Now, while Harry started his army, of course, and began the training, I don’t think that there’s any doubt that Neville was leading them while Harry was off hunting Horcruxes and killing Voldemort. He helped train them so they could defend themselves, he kept their spirits up and refused to bow to the Carrows, he was the symbol Harry couldn’t be because Harry wasn’t there.
In short, I think Neville represents all the unsung heroes who make the known heroes actions possible.