C.S. Lewis's lifelong nickname was Jack. His birth name was Clive Staples Lewis, but after a childhood incident involving the death of a dog named Jacksie, he began calling himself Jack and retained the name.
Lewis's family and close friends used Jack, while C.S. Lewis became the public name attached to his scholarship, fiction, and Christian writing. His brother Warren, usually called Warnie, was among the people who knew him by the nickname throughout his life.
The detail can cause confusion because Lewis's legal name and literary name are both widely cited. Jack was not a pen name and was not a character from the Narnia books. It was the personal name he chose for everyday use, making it distinct from the initials on his published works.