Tom Riddle aka Lord Voldemort

Tom Marvolo Riddle was a half-blood wizard whose father abandoned his wife and unborn son. His mother died shortly after childbirth, leaving Tom an orphan. He hated the other children and used special powers he'd discovered to torment them. Albus Dumbledore came to take a look at him and told Tom he was a wizard and a Parselmouth. He took Tom to the wizarding world to attend Hogwarts.
He was a brilliant student, but was secretly power hungry, looking for a way to gain more control. He discovered Horcruxes, and made one to "preserve" his soul. He then released a Basilisk from the Chamber of Secrets. It terrorized the school and eventually killed a student. With Hogwarts about to be closed, Tom framed another student, Hagrid, and his pet Acromantula for the death and frights. Hagrid was expelled and the Acromantula was driven into the forest.
Tom grew into a strong young man with a natural skill for magic. He continued to make Horcruxes. Eventually, he had risen to great power in the wizarding world. He went to kill the Potters and their son. (One of two boys that may fulfill a prophesy to kill him.) He killed James and Lily, but when he used the Killing Curse on Harry it backfired and killed him. (Although he wasn't "dead." His Horcruxes kept his sould bound to Earth.) It took a long time and more killing before Voldemort was brought back into the living realm.
He is in power once again and hunting for Harry. Can Harry destroy all of the Horcruxes and finally get rid of Voldemort? Or will Harry be the one that gets killed? Find out in the new Harry Potter and the Deathly Hallows Movie!

Hermione Granger

Hermione Granger was one of the most skilled first-year students, despite the fact that she had no magical parents. She is intelligent, and a skilled witch. She was sorted into Gryffindor, even though the Sorting Hat thought to put her into Ravenclaw.
Along with Harry and Ron, she skipped her last year at Hogwarts to hunt down the remaining Horcruxes. (You can watch this in the new Harry Potter and the Deathly Hallows movie, coming out soon.)
She eventually marries Ron and they have two children, Rose and Hugo.

Ronald Weasley

Ronald Weasley is Harry Potter's best friend. He met him on the train to Hogwarts for the first time. They became fast friends. Ron was sorted into Gryffindor. He is the only pure-blooded wizard in the trio of Harry, Hermione and Himself. He is headstrong and always getting into trouble. He is brave though, and fiercely loyal.
He skipped his last year along with Harry and Hermione to destroy Voldemort's Horcruxes. (This adventure will be released soon in the Harry Potter and the Deathly Hallows movie.)
He has always been the funny one, and loves to crack jokes. Hermione was frequently frustrated with Ron's immaturity, once commenting that he had the "emotional range of a teaspoon." In the search for the Horcruxes though, he matured a lot, even leading the team when Harry was debilitated.