Harry Potter and the Deathly Hallows
Affiliation: Dumbledore's Army (D.A.)
House: Gryffindor, 7th Year
Neville really comes into his own here – he's a far cry from the dopey, forgetful, clumsy boy we first met in Book 1. Here, he's confident, rather swashbuckling, and bold, and he's been a leader of the rebel forces of Hogwarts all the time that Harry, Ron, and Hermione have been away.
Neville's role in Deathly Hallows may be brief, but it reminds us that it's not all about Harry; Neville's been keeping the troops ready to act, and his struggle, which we hear about a bit but don't see, is just as important in some ways. Remember, Neville and Harry's fates are somehow tied – Neville was the other boy who might have been the "Chosen One," according to the old prophecy. And, appropriately, he now proves himself to be just as brave and bold as the Boy Who Lived. We can't help but cheer when Neville bravely faces Voldemort and strikes the head off of Nagini, Voldemort's Horcrux-snake – finally, Neville has come into his own and become a hero! His grandma must be very proud.
In the Epilogue, we learn that Neville has become Professor Longbottom, esteemed teacher of Herbology at Hogwarts. After Deathly Hallows was published, Rowling also had this to say about Neville:
To make him extra cool he marries the woman who becomes, eventually, the new landlady at The Leaky Cauldron, which I think would make him very cool among the students, that he lives above the pub. He marries Hannah Abbott. (source)