Agatha

Yaroslav the Wise definitely had at least four daughters. Three are known to history, as are the important figures they married. The fourth daughter remains obscure, but is thought by some scholars to have been the mysterious "Agatha", wife of Edward the Exile of England.

Agatha's origin is alluded to in numerous surviving medieval sources, but the information they provide is sometimes imprecise, often contradictory, and occasionally cannot possibly be correct. The earliest surviving source, the Anglo-Saxon Chronicle, along with Florence of Worcester's Chronicon ex chronicis and Regalis prosapia Anglorum, Simeon of Durham and Ailred of Rievaulx describe Agatha as a kinswoman of "Emperor Henry" (thaes ceseres maga, filia germani imperatoris Henrici). In an earlier entry, the same Ailred of Rievaulx had called her a daughter of emperor Henry, as do later sources of dubious credibility such as the Chronicle of Melrose Abbey, while Matthew of Paris calls her the emperor's sister (soror Henrici imperatoris Romani). Geoffrey Gaimar in Lestoire des Engles states that she was daughter of the Hungarian king and queen (Li reis sa fille), although he places the marriage at a time when Edward is thought still to have been in Kiev, while Orderic Vitalis in Historiae Ecclesiasticae is more specific, naming her father as king Solomon (filiam Salomonis Regis Hunorum), actually a contemporary of Agatha's children. William of Malmesbury in De Gestis Regis Anglorum states that Agatha's sister was a Queen of Hungary (reginae sororem) and is echoed in this by Alberic of Trois-Fontaines, while, less precisely, Ailred says of Margaret that she was derived from English and Hungarian royal blood (de semine regio Anglorum et Hungariorum extitit oriunda). Finally, Roger of Howden and the anonymous Leges Edwardi Confessoris indicate that while Edward was a guest of Kievan "king Malesclodus" he married a woman of noble birth (nobili progenio), Leges adding that the mother of St Margaret was of Rus royal blood (ex genere et sanguine regum Rugorum). https://en.wikipedia.org/wiki/Agatha,_wife_of_Edward_the_Exile

More commentary on Agatha: https://fasg.org/projects/henryproject/data/agath000.htm (path correction)