Who is Count Gerold of Villaris from Pope Joan and what is their importance?

Gerold is a red-haired knight who first sees Joan when she meets the bishop of Dorstadt, arriving to attend school. Although there is a considerable age difference between them, Gerold falls in love with her, and is her best friend as she grows up. Gerold is a smart man, a soldier and also a civil engineer, directing huge building projects in Rome, including the Leonine Wall, as well as the restoration of the aqueduct. Having lost his entire family and his estate, Gerold returns to his military career, fighting battles for Lothar, the emperor of Europe.