Coarse, snappy, and slightly prudish, Professor McGonagall has nonetheless shown herself to have the best interests of the students of Hogwarts, of her wards in Gryffindor, and especially of Harry himself in mind. McGonagall is also one of Albus Dumbledore's staunchest supporters in the Order of the Phoenix.
McGonagall was played by Maggie Smith in the films.
We are first introduced to Minerva McGonagall in the opening chapter of the Philosopher's Stone, The Boy Who Lived, when she meets Albus Dumbledore at Number 4 Privet Drive. This is when we find out that Minerva McGonagall is an animagus, a witch or wizard who has the ability to turn into an animal without the aid of a wand, as, according to her, she sat on a brick wall all day in the form of a tabby cat. In this chapter, Professor McGonagall's complete personality is summarised: her caring side, (she is worried when Dumbledore reveals he plans to leave Harry Potter, fresh from Lord Voldemort's attack on him, with the Dursley family, his only living relatives), her snappy, brusque side (she criticises a number of people, including Hagrid), and ultimately, her intelligence (she is revealed to be an animagus; to become one of these is apparently very difficult, and can sometimes take years to master the magic).
McGonagall is the member of staff who performs the sorting ceremony at Hogwarts. She commands complete respect from all of her students.
McGonagall is very keen on Quidditch, and, as Head of Gryffindor House, Professor McGonagall takes an interest in the house Quidditch team's progress. In Harry Potter and the Philosopher's Stone, she recommends Harry as seeker.
Professor McGonagall is a member of the Order of the Phoenix, as is revealed in Harry Potter and the Order of the Phoenix. She warns Harry not to provoke Dolores Umbridge, as she works for the Ministry of Magic. In that book, she also attempts to keep Dolores Umbridge and her minions from taking Hagrid away forcefully, and pays for her attempt when she gets hit by four Stunning Spells. She is taken from the school to St. Mungo's. However, after the battle in the Department of Mysteries, she returns to the school safe and well, returned to her usual brisk self.
McGonagall has black greying hair drawn into a tight bun. She wears emerald green robes, and always a very prim expression. She is, according to Rowling "about 70 years old".
Her name derives from two sources: one is from Minerva, the Roman Goddess of Wisdom (so this name suits Minerva very well). Her surname comes from that of a 19th century Dundee eccentric, William Topaz McGonagall. He is often regarded as the worst poet in the history of the English language.
Reference
Rowling, J. K. Harry Potter and the Order of the Phoenix. ISBN 0-439-35806-X