Magna Charta Sureties does not give parents for Agatha. There is a lot of doubt as to her parents are. I have the "classic" ancestor, but others (including Vajay) say it is Liudolf, Margrave of Westfriesland and Gertrudis, daughter of Hugh IV of Egisheim (whom I name Ludwig von Braunschweig, Count of Derlingau & Gertrude von Egisheim).
Cannon and Griffiths, The Oxford Illustrated History of the British Monarchy, describes her as the niece of Henry II, Emperor of Germany (Holy Roman Emperor). As Henry's sister Gisela married Stephen of Hungary, the ancestry that I have would fit that description. Of course Vajay uses the "niece" datum to portray a different ancestry.