Who is Gudrun and Matthew of Ingleheim from Pope Joan and what is their importance?

Gudrun, Joan's mother, a blond Saxon, is rescued from hunger after an attack on her village by Karolus by the missionary John, who later becomes Canon of Ingleheim. She loves the mythical fairy tales and stories of her youth, and shares them privately with her one daughter, Joan, against the wishes of her husband. Her childhood beliefs are considered pagan and evil in the culture she is forced to adopt, but she never forgets the songs, stories and characters.