Severus Snape
SPOILERS: This page contains information related to ALL six books
Hogwarts Potions Master and Head of Slytherin House, Snape has always been an obscure character. Despite having all Dumbledore's trust, he always seems to be hovering close to the dark side. Envious of the Defence Against The Dark Arts postion that Dumbledore has always denied him, Snape isn't really that appreaciated by many at Hogwarts.
Despite being Half-Blood - his mother a witch and his father a muggle, Snape is a true Slytherin and acts just like one. He is unfair, favors his students and taunts the weak as much as his position as a Professor will allow him. Many students are afraid of him - such as Neville - and he had a tendency to induce mini break-down amoungst his students during tests.
Snape attended Hogwarts as a student at the same time as Harry parents. James and Sirius hated him - much like Ron and Harry hate Draco - and would regularly taunt him (the grey underpants scene in OttP springs to mind!). In Harry, Snape sees James once again and delights in taunting the son as he could not the father.
Obsessed by the Dark Arts, Snape joined the Death Eaters as Voldemort gained power. He was sent to Hogwarts to be a spy and overheard the prophecy concerning Harry and Voldemort. He ran and tols his master and is therefore directly responsible for James & Lily's death. Realising what he'd done, he apparently repented and came to Dumbledore. Dumbledore believe him to be truthful and used him to spy on Voldemort. Now a member of the Order of the Pheonix, Snape remained amoung Dumbledore's closest for many years.
When Voldemort rose again, Dumbledore sent Snape back to his master, hoping he'd be a double agent once more.
After reading Book six, the first impression is that Snape fooled Dumbledore into believing he'd turned good when he had infact only been loyal to Voldemort. When Snape kills Dumbledore this thoery seems confirmed, but I really hope there is more to it...