Who is Rolf Linden from A Test of Wills and what is their importance?

Rolf Linden is deceased. He was a German prisoner of war who came to help Catherine Tarrant on her estate farm during World War I. He was strong and capable and good, and they fell in love. But their relationship ended abruptly when, after she'd written a letter of appeal to Charles Harris, the authorities took Linden away. Catherine didn't find out that he'd died of influenza until almost a year later.