Who is Mr. Kirkpatrick from Surprised by Joy and what is their importance?

Mr. Kirkpatrick, or the Knock, or the Great Knock, was a tutor of the boys' father when he was young, and then was an invaluable teacher to both the Lewis boys. C.S.'s older brother went to him when it was revealed that he was not learning at Wyvern, and Lewis followed as a result of the same fact, and thrived under the tutelage of a man who was determined to teach him to think and communicate clearly and logically, learn other languages, and become an independent learner.