Who is Christopher Tolkien in The Letters of J.R.R. Tolkien by J.R.R. Tolkien?

Christopher Tolkien is the author's son. He is listed as having assisted in the arrangement of the selected letters that comprise this book. He shows up mainly as a recipient of letters throughout the course of the book. There are a variety of types of letters written to Christopher. There are regular letters. There are also airgraphs. He even wires his son. The impression is that they found one method of closeness by maintaining quite a lengthy correspondence with one another.