In the original novel, the Good Witch of the South is Glinda.
Glinda first appears in L. Frank Baum’s 1900 book, The Wonderful Wizard of Oz. She rules the Quadling Country, the southern region of Oz, and is presented as a powerful and benevolent sorceress. Dorothy and her companions travel south to seek Glinda’s advice after the Wizard leaves the Emerald City.
Glinda helps Dorothy discover how to return to Kansas using the Silver Shoes. She also arranges for the Winged Monkeys to escort the Scarecrow, Tin Woodman, and Cowardly Lion safely home. In later Oz books, she becomes an important protector of Princess Ozma and a central magical authority.
A common mix-up comes from the famous 1939 MGM film. That adaptation identifies Glinda as the Good Witch of the North and combines her with the unnamed Good Witch who greets Dorothy in Munchkinland. The novel’s Glinda, however, is explicitly the Good Witch of the South.