He is a plump, bumbling, Gryffindor student in Harry's year, with a perpetually bad memory. He and Harry share a dormitory with Ron Weasley, Seamus Finnigan, and Dean Thomas.
Neville's best subject is Herbology and his worst is Potions. His greatest fear is of the Potions Master, Professor Severus Snape. He owns a toad named Trevor, who was a gift from his great uncle Algie upon his acceptance to Hogwarts School of Witchcraft and Wizardry.
Neville's parents are Alice and Frank Longbottom. They were both Aurors and prominent member of the Order of the Phoenix during the time of Lord Voldemort's original reign. They were driven mad by followers of Voldemort, and are now in a closed ward of St. Mungo's Hospital for Magical Maladies and Injuries. Neville's grandmother raised him from a young age.
The driving mad of his parents affected Neville greatly. Until the events of Harry Potter and the Order of the Phoenix, Harry Potter was his only schoolmate aware of their situation, and Albus Dumbledore made Harry promise not to tell anyone. It could be that much of Neville's uncertainty stems from events concerning his parent's madness.
Neville's lack of self-confidence could also stem from his weakness as a wizard-in-training; despite being pure-blooded, Neville shows an ineptitude with magic that often gets him into trouble (though he shows great strength and potential, being able to whiz the Charms teacher, Professor Flitwick, around the classroom, he lacks focus). Nonetheless, he has stood up both to his friends (notably in Harry Potter and the Philosopher's Stone, where his bravery earned Gryffindor the House Cup) and his enemies (see Harry Potter and the Order of the Phoenix for details). Indeed, during the course of the fifth book, Neville's magical abilities improve dramatically, as shown in the renegade Defence Against the Dark Arts lessons taught by Harry. This may stem from his desire to stop Lord Voldemort, due to the actions of the evil wizard's minions.
Neville is also known for his forgetfulness; he used a Remembrall in the early books of the series to help him recall things, but couldn't remember what it was reminding him of.
Despite a portrayal that could be described as "one-sided" in the first four books, Neville has come into his own, particularly in the fifth book. Indeed, he plays a key role (albeit inadvertently) in keeping Lord Voldemort from knowing the prophecy made about Voldemort and Harry.
An interesting point which Dumbledore brings up in the denouement of Harry Potter and the Order of the Phoenix is that both Harry and Neville were born during the same month, and the aforementioned prophecy could possibly have applied to either of them. Whether this is pertinent to the remainder of the story is yet to be determined.
References