Derek Hale

Derek Hale is a dark and mysterious character whose past, secrets, and motives remain unknown. He is currently Scott's mentor, teaching him the fundamentals of being a Werewolf. He is physically very strong and can be sadistic and cruel, but he is loyal to his kind. It was thought that during a full moon, he bit Scott, but later in Episode Three (Pack Mentality,) it is revealed he didn't. The werewolf responsible is known as the Alpha Wolf.

Derek's whole family was killed in a fire, though his sister was recently found dead and bisected. She was killed by an Alpha she was tracking. It is interesting to note that they both seem to be different types of werewolves: Lauren, as an Alpha, had the ability to completely change into a full wolf form, while Derek, as a Beta, is more human-like. According to the bonus clips from MTV, Derek is a born werewolf.

It would appear that being a werewolf by birth or by bite does not automatically determine what shape they will eventually shift into. Although logically, there are more Betas than Alphas. What determines which shape a werewolf will eventually take, however, is uncertain at this point, as is the concept of whether a Beta can eventually become an Alpha and thus change their shape into a more bestial form. Allison´s father seems to be familiar with Derek, his family having possibly slautered his relatives. With Lauren now deceased, Derek is the only known surviving member of the Hale 'pack,' other than an uncle who was left in a comtose state.

In Magic Bullet, Derek's arm was infected with Nordic Blue Monkshood, a strain of Wolfsbane that was fired at him by Kate Argent. It was designed to end his life within a period of 48 hours. Stiles nearly had to aputate it, much to his chagrin, but Scott thankfully arrived with the very bullet that had previously been shot into the arm. Derek then lit the contents of the bullet on fire and injected them into his arm, causing him great pain, but reversing all effects of the projectile.

Quotes
Either you start the car or I will rip your throat out...with my teeth.

--Derek to Stiles in Magic Bullet