Who is Colin Lindsey from The Jewel in the Crown and what is their importance?

Colin is one of Hari Kumar's friends from their time as students in Britain. After graduation, Colin joins the army and is stationed in Mayapore, India, which is Hari's home. By the time Colin arrives in India the two are no longer close..... Colin not only avoids Hari..... he goes so far as to pretend he doesn't know him.