Who is Catherine Tarrant from A Test of Wills and what is their importance?

Also known as C. Tarrant, the renowned artist, Catherine Tarrant is a young woman who lives on one of the estates near Upper Streetham. Her involvement in the case becomes suspicious when she gives Hickam money and he promptly drinks himself almost to death. In addition, her shadowy past is revealed when it comes out that she had a love affair with a German prisoner of war. She asked Charles Harris, via a letter from Lettice Wood, to help her with her case because she and her German, Rolf Linden wanted to be married. Instead Rolf was taken away and she found out much later that he died of influenza in London. Catherine, who is dark-haired and attractive, was once engaged to Mark Wilton, to whom she still owes loyalty. Rutledge is drawn to her art, and to her inner pain. The town shuns her for her involvement with the enemy.