Roman the Great
Probably between the end of 1149 and 1151, Agnes married Prince Mstislav Iziaslavich of Pereyaslavl, the eldest son of Grand Prince Iziaslav II of Kiev. The Chronicler Wincenty Kadłubek, who knew the Piast-Rurikids affinities, explicitly described in his "Chronica Poloniae" that Agnes was given to Mstislav as a wife. Further confirmation of this fact is that Mstislav's eldest son was called "nephew" of Casimir II the Just, and the relationship existing between Roman the Great and Leszek the White is described as the second-degree cousins. In addition, Roman is named "jątrew" (wife's brother) of Leszek in the Hypatian Codex. Therefore, if Mstislav was to marry any of Bolesław III's daughters, the only one he could marry was Agnes.