Kievan theory
Jetté pointed out that William of Malmesbury in De Gestis Regis Anglorum and several later chronicles unambiguously state that Agatha's sister was a Queen of Hungary. From what is known about the biography of Edward the Exile, he apparently was in the political faction of Andrew I of Hungary, following him from Kiev to Hungary in 1046 and staying at his court for many years. Andrew's wife and queen was Anastasia, a daughter of Yaroslav the Wise of Kiev by Ingigerd of Sweden. Following Jetté's logic, Edward's wife was another daughter of Yaroslav. In so doing, he mirrored a solution to Agatha's paternity put forward in 1894 by William Ferrand Felch.
This Felch/Jetté theory accords with the seemingly incongruous statements of Geoffrey Gaimar and Roger of Howden that, while living in Kiev, Edward took a native-born wife "of noble parentage" or that his father-in-law was a "Rus king".
Eduard Hlawitschka also identifies Agatha as a daughter of Yaroslav, pointing out that Adam of Bremen, who was well informed on North-European affairs, noted around 1074 that Edward was exiled in Kievan Rus (E[d]mund, vir bellicosus, in gratiam victoris sublatus est; filii eius in Ruzziam exilio dampnati) and that the author of Leges Edwardi confessoris, who had strong ties with Agatha's children, Queen Margaret of Scotland and her sister Cristina, and could thus reasonably be expected to be aware of their descent, recorded around 1120 that Edward went to the land of the Rus and that there he married a noblewoman.
Onomastics have been seen as supporting the Kievan theory. Among medieval royalty, Agatha's rare Greek name is first recorded in the Macedonian dynasty of Byzantium; it was also one of the most frequent feminine names in the Kievan Rurikid dynasty. After Anna of Byzantium married Yaroslav's father, he took the Christian name of the reigning emperor, Basil II, while some members of his family were named after other members of the imperial dynasty. Agatha could have been one of these.
The names of Agatha's immediate descendants — Margaret, Cristina, David and Alexander — likewise extraordinary for Britain at that time, have also been suggested as clues. The names Margaret and Cristina are today associated with Sweden, the native country of Yaroslav's wife Ingigerd. The name of Margaret's son, David, mirrors that of David of Hungary, like his elder brother Solomon a son of Andrew I of Hungary and Anastasia of Kiev. Furthermore, the first saint of the Rus (canonized ca. 1073) was Yaroslav's brother Gleb, whose Christian name was David.
The name of Margaret's other son, Alexander, may point to a variety of traditions, both occidental and oriental: the biography of Alexander the Great was one of the most popular books in eleventh-century Kiev, and it was a common name in the Greek-influenced Orthodox tradition. Humphreys would review the two main types of hypotheses, which he called the Salian and the Slavic theories, and pointed out that the critical evidence in weighing them is whether one accepts the testimony of John of Worcester (Salian) or William of Malmesbury (Slavic) as representing the earliest, most accurate version of her ancestry. He would later point out the occurrence of the name Maria in the next generation of the Kievan dynasty, and suggested that Agatha could instead have been sister of the Byzantine wife of Vsevolod I of Kiev, that the tradition of imperial connections had confused which Empire was involved. However, he subsequently studied the sources and particularly the chronology of this dynasty in more detail and concluded that this solution was unlikely, though he did favor a reconstruction making Yaroslav the son, rather than the step-son, of the Byzantine princess Anna Porphyrogeneta.
One inference from the Kievan theory is that Edgar Ætheling and St. Margaret would have been, through their mother, first cousins of Philip I of France. The connection seems too notable to be omitted from contemporary sources, yet there is no indication that medieval chroniclers were aware of it. An argumentum ex silentio regarding this relationship has been a factor leading critics of the Kievan theory to search for alternative explanations.
