Minerva McGonagall is a fictional character in the Harry Potter books written by J. K. Rowling. This character is a teacher at the school Hogwarts, and a good character in the series; she helps Harry Potter, the main character, many time in the series. She teaches a subject called "transfiguration", this a type of magic where you change objects or animals into other objects and animals. All the way at the end of the books, McGonagall becomes the school principle, her honorary title is called Hogwarts Headmistress.
When McGonagall was young she went to Hogwarts too, where went to Griffindor, a house for brave and bold students. When she got older she decided to train herself to be an "animagus", that is a wizard who can transform themselves into an animal without a wand, now she can transform into a cat. Later, she became a member of the Order Of the Phoenix, a good organization trained to fight evil wizards.
She is played by Maggie Smith in the movies.