Andrew Ketterley was a magician, or at least a practitioner of magic, living in London around 1900. He was the uncle of Digory Kirke, and one of the first humans to set foot in Narnia.
Andrew's godmother was a woman known as Mrs. Lefay, and although he was very fond of her, he was not encouraged to see her, as she was viewed to be insane. Mrs. Lefay openly claimed to be a fairy, and told her godson that she knew of the existence of other worlds. In time, the two were separated when she was sent to prison.
Andrew was asked to come see Mrs. Lefay in her last days. Before dying, she gave to him a box of magical origin, which she made him swear to burn with certain (probably magical) ceremonies. Though Andrew swore to do this, he did not keep his promise. Instead, he embarked on a long career in magic.
Knowing the box and its contents to be of magical origin, Andrew set out to discover how to use the contents of the box. This search took him most of his adult life. During this time, Andrew apparently had no career, and instead preferred to live off the charity of his sister Letitia. He also searched and researched forms of magic, and eventually became quite an adept 'magician'.
After years of research, he discovered the box to be from the island of Atlantis. Inside, there was dust from another world. Upon discovering this, Andrew began to work in his study, working at using the dust to access that other world.
This went on until Andrew was in his 60s. At that time, his sister Mabel grew extremely ill. She and her son came to stay with Andrew and Letitia in the summer of 1900. During that summer, Andrew conducted his first 'successful' experiment with the magic dust; he invented a series of rings to bring a person to or from the other world, and began sending guinea pigs out of Earth and into the other world. After succeeding, Andrew began looking for a human subject for his experiments.
He found it in the form of his nephew, Digory Kirke, the son of his ailing sister Mabel. Digory, at one time, stumbled into his uncle's study with a girl by the name of Polly Plummer. After locking them in his study, Andrew tricked Polly into taking a yellow ring (designed to take her out of earth and into the otherworld), but not a green ring (designed to bring her back). However, he did not know that a person did not have to be wearing the rings to be brought back. After sending her into the other world, he told his story to young Digory and gave the boy an ultimatum: Allow Polly to meet an unknown fate in another world, or go after her himself.
After arguing with his uncle, Digory chose to intervene and go to the other world himself. When Digory returned, he brought not only Polly back with him, but also a stranger; a queen from the world of Charn, called Jadis. Jadis, herself an extremely powerful witch, at once dismissed Andrew as an impotent magician. Andrew was instantly smitten, though his attraction quickly waned, as he was forced to come with her, as she attacked his sister, stole a cab and cabhorse, robbed a jewelery store, and proceeded to make mayhem in London.
As they returned to his sister's residence, police were called and began to attempt an arrest. Andrew, Digory, and Polly were quickly caught in the crossfire, as Jadis clashed with the police and the populace of London. In the midst of the 'Battle at the Lamppost', Digory and Polly managed to drag Jadis back into the otherworld by means of their uncle's magic rings. Andrew, the stolen cabhorse, Strawberry, and its owner were all dragged along as well, in the effort to take Jadis back to her own world.
Instead, all six were dragged into an unfamiliar world still in its birth. One of their first encounters was a giant Lion, still singing the world into existence.
Andrew was instantly terrified, as well as slightly drunk, and attempted to run, but instead tripped into a brook and fainted. There he was found by animals living in the new world.
Capable of thought and speech, these animals discussed what to do with Andrew. In time, they made him a cage, and kept him there as a sort of pet, whom they nicknamed 'Brandy', because that was the one word he was able to utter in his confusion.
Several days later, a terrified Andrew was brought before the great Lion Aslan, and made to sleep and forget all the things that had frightened him. He, Digory, and Polly returned to Earth.
When Digory and his parents moved to their new country estate, Andrew was taken to live with them, but he had learned his lesson. He never tried any magic again in his life, and in his old age he became a nicer and less selfish old man than he had ever been before. However, he had a constant habit of getting visitors alone in the billiard-room and telling them stories of his encounter with Jadis (though he only described her as a foreign royal with a devilish temper. "A dem fine woman", as Uncle Andrew put it.
Andrew is a man who seeks importance and recognition, and is not afraid of hurting other people to get it. At the beginning of the book, he is extremely selfish, pompous and cruel. When in the presence of his superiors, he flatters and grovels. Anyone who is not his superior, he considers to be below him. When threatened, he tends to panic and fall apart. He also has a tendency to drink, which can hinder his usefulness in times of need.
Andrew seems to believe that as a magician, he is not subject to the laws and moral codes that bind other humans. He is obsessed with magic, and the opportunities it will afford him. He is considered by others to be eccentric and slightly crazy because of his beliefs.
Throughout his life, Andrew is portrayed as a small, incompetent man trying to be someone, and desperately failing at all the critical moments of his life. Unlike the man he aspires to be, he never learns his lessons, and never achieves greatness. As an old man, however, he becomes kinder and less selfish, likely due to the shock of having unintentionally witnessed Narnia's creation and, although Aslan removed his memories, Andrew realised how misguided and fruitless his obsession with magic was, and he gave it up to enjoy his family.