Severus Snape

Professor Severus Snape, Head of Slytherin is a wizard teaching Potions at Hogwarts as of 1993.

Past
Professor Snape's past up until 1991 is identical to his canon self.

1991-1992
In Summer of 1991, Professor Dumbledore asked Professor Snape to help protect the Third Floor Corridor (then hiding place of the Philosopher's Stone). Snape achieved this through a Potions riddle.

Early on in the school-year, Snape developped a great animosity against one of the first-year students, Harry Potter, because of his past enmity with Harry's father James. At the same time, Snape had secretly been tasked with protecting Harry from harm, especially coming from Quirinus Quirrell and his evil turban. Hermione Granger also attended his class, and, thanks to having read ahead, performed marvelously, although she still innerly objected to his bullying ways (snarikly noting he "obviously thought teaching kids worked like training attack dogs").

In October, Snape was caught up in a prank from Fred and George Weasley against their brother Ron: they managed to get Ron's monthly Quidditch magazine forwarded to Snape, notorious Quidditch hater, who destroyed the issue on sight, much to Ron's dismay. In retaliation, Ron arranged for Snape to give his mischievous brothers detention over an exploded cauldron that Ron himself had sabotaged.

During that year's Halloween Feast, after Quirinus Quirrell ran the alarm concerning a troll roaming through the Dungeons, Albus Dumbledore ordered the Heads of Houses to escort their students to their Common Rooms. Snape objected by pointing out the Slytherin Common Room was, in fact, in the Dungeons. Thus, he stayed in the Great Hall, giving him the opportunity to back up Dumbledore's Stunning Spell when the troll erupted into the Great Hall while chased by Peeves.

During the Christmas holidays, Hermione and Harry speculated on the possible reasons behind Snape's callous behavior towards Harry. Hermione suggested Snape might have been a friend of Harry's father, disappointed with Harry for not living up to Snape's memories of his former friends — a clever but spectacularly wrong theory.

Later that year, Snape almost noticed Hermione Granger speaking Parseltongue to the Great Basilisk hidden inside the wall. However, Hermione stopped hissing immediately and her potion was entirely accurate. Finally, when Harry Potter, Hermione Granger and the Great Basilisk breached the Third Floor Corridor in pursuit of Quirrell and Tom Riddle, Snape went in after them an hour or so later. The Basilisk had already departed and the two children omitted her from their tale, but it still made quite an impression on Snape. The Potions Master also informed them of Tom Riddle actually being Lord Voldemort, which they did not know.

1992-1993
Before the start of the school-year, Professor Snape was once again recruited by Professor Dumbledore in order to update the protections on the Third Floor Corridor. This time, Snape's trial was the first one. He put a seal of Queenstinger Wax on the trapdoor, and left ingredients for a potion that could dissolve the seal.

On Spider Day, Severus Snape was present during the Centaurs' initial parlays with Albus Dumbledore, although he stayed silent. Later,he participated in the battle against the Acromantula soldiers along with the other Hogwarts Professors.

During the first second-year Potions Class following the Christmas holidays, Professor Snape assigned his student to brew a Shrinking Solution, which Hermione Granger attributed to his being in a particularly foul mood that day, as Shrinking Solutions were normally third-year material. After Snape threatened Neville Longbottom to feed his faulty solution to his toad Trevor, Maximilian Candy stepped in and offered to test the potion himself. Snape was unable to stop him, and aghast when the potion appeared to work entirely accurately despite its obvious wrongness. Hermione, aware of what had actually happened (namely Maximilian using his shapeshifting abilities to fake the results), laughed at Snape's surprised expression. Because of this and of Maximilian's hot-headedness, Snape deducted seventy points in all from Gryffindor House.

By the next lesson, where he assigned his students to brew a remedy for Vermicious Pawpox in Kneazles, Hermione had finalized a plan to get back on Snape for all the unfair punishments he'd ever given out, particularly to her, Harry and Neville. Using various wording tricks, she managed to trick Professor Snape into accidentally taking fifteen thousand points from Slytherin House. (That very day, she'd arrange for Albus Dumbledore to put up a limit on how many points could be given to a House every month, preventing Snape from cancelling the losses.) She then forcefully fed him a draught of snooziness brewed by Harry and Ron.

1993-1994
When Sirius Black's "ghost" appeared in the Great Hall on Halloween, Professor Snape was naturally quite dismayed, especially as Sirius began targeting him in particular. Outright terrified by the end of it, Snape ended up fleeing the Great Hall using Unsupported Flight (an ability he had previously demonstrated before Hermione Granger at the tail-end fo the previous year).

Personality
Professor Snape is above all a bitter and unhappy man. Haunted by the mistakes of his past and by grudges he refuses to give up, Snape often takes his nerves out on an unsuspecting class of first- or second-year students, scaring and taunting and punishing. Loathed and feared by most, Snape seems to care very little for what others think of him, but demands outwards respect from his student no matter how they truly feel.

Although some of it may only be a façade put up to impress upon remaining Death Eaters that he is no traitor, so as to be able to once again be a spy for Professor Dumbledore in the event of Lord Voldemort's return, Snape greatly favors Slytherin House in his duties as Professor, and in turn bullies Gryffindors mercilessly, much moreso than any others.