Who is Captain Mark Wilton from A Test of Wills and what is their importance?

Captain Mark Wilton is a flyer, a wartime pilot, and a decorated hero, having received the Victoria Cross for his actions during World War I. He is acquainted with the King and the Royal family, and his own family members are wealthy bankers and landowners. He is engaged to Lettice Wood, Colonel Harris's ward. He lives with his cousin, Sally Davenant, at her manor house near Upper Streetham. Before the war, he was briefly engaged to Catherine Tarrant, but that relationship ended when he entered service at the beginning of World War I. He is tall, strong and handsome, a perfect poster boy for heroism and the early days of flying aces. Wilton is the primary suspect in the case, with the motive, means, and opportunity to have killed Charles Harris that morning.