Minerva McGonagall.
Spoilers may be included in this section.
Minerva McGonagall is Deputy Headmistress, head of Gryffindor House, Transfiguration professor, and later Headmistress at Hogwarts, where she began teaching in December 1956. McGonagall considers Transfiguration to be amongst the most complex and dangerous magic taught at Hogwarts. McGonagall is first introduced in the opening chapter of Harry Potter and the Philosophers Stone, when she meets Albus Dumbledore at Number 4 Privet Drive, Little Whinging, Surrey (the home of Harry Potters aunt and uncle).
McGonagall is described as a tall, rather severe-looking woman, with black hair typically drawn into a tight bun, only letting her hair down at the Yule Ball. She wears emerald green robes, a pointed hat that is cocked to one side, and always has a very prim expression. She is, according to Rowling, a "sprightly" 70-year-old. McGonagall wears square spectacles that match the markings around the eyes of her Animagus form of a silver tabby cat; she has the distinction of being the first Animagus introduced in the series and as one of the few registered (legal) Animagi of the century. Harrys immediate impression of her is of someone who is not to be crossed. Rowling has said McGonagalls birthday is October 4.
McGonagall is very keen on the success of the house Quidditch team. In the first book, after seeing Harry fly masterfully his very first time on a broom, she recommends him to fill the position of Seeker, even though first years are normally disallowed from playing. Furthermore, although a strict disciplinarian, she often assists Harry indirectly with activities that are not strictly within the rules of Hogwarts; for example, she allowed Harry and his friends to use the Transfiguration classroom to practice for tasks in the Triwizard Tournament. Despite her stern front, McGonagall has been known to display a range of emotions, which can often be a shock to her colleagues and students. Indeed, Harry claims that one of the worst sounds he ever heard was Professor McGonagalls scream at the end of Harry Potter and the Deathly Hallows on finding out Harrys apparent death.