Who is His Brother from Surprised by Joy and what is their importance?

Lewis' brother is never named in the book, but is his primary and closest companion, particularly in their childhoods. The boys shared a passion for the imaginary, and that haven became even more therapeutic and necessary when their mother died in their early childhood. His brother was a lover of all things to do with India, and so while Lewis was populating his imaginary world with anthropomorphized animals in clothes and calling it Animal-Land, his brother was appointing Rajas and imagining richly appointed castles and tents.