Esterka was a legendary Jewish mistress of Casimir the Great, the King of Poland between 1333 and 1370. She was the daughter of a poor tailor from Opoczno named Rafael. The King had several wife's, but Esterka was the only one who gave him male offspring despite the fact that they never were officially married. Esterka had played a significant role in Casimir's life. She was legendary beautiful and intelligent women who performed as a king's adviser in support of varies initiatives: building stone cities, tolerance to representatives of different religious faiths, free trade and support of cultural development.It was her who innisiate to built a foundation for a tolerant attitude towards Jews in Poland and it was laid for centuries, making Poland into "a paradise for the Jews".
Casimir was loyal to the Jews, and encouraged them, as a result the country experienced fenominal economic and cultural growth. Casimir was called The Great King for his wiseness and bright vision. The sons of King Casimir and Esterka Pelko and Nemir were baptized on the request of thair father and became the ancestors of several Polish noble families. To develop legal and commercial relations between Jews, Poles and Germans, Pelko was sent to Konitz and his brother Nemir in 1363 to the southwest (Lower Silesia) to participate in the foundation of the city of Neurode, which later became the patrimonial nest of the new noble Rudanovsky dynasty. 
The earliest historical record of Esterke found in Jan Długosz chronicle 1386, where he writes that Esterka influenced Casimir the Great's policy in addressing the Jewish issues:
«Ad preces quoque praefatae Ester judeae et concubinae, exorbitantes praerogativas et libertates per literas singulis judaeis in regno Poloniae habitantibus, quae falso scriptae ab aliquibus insimulabantur, et quibus divina majestas contumeliatur et offenditur, concessit, quarum factor olidus etiam in hane diem perseverat»
Several places, streets and monuments in Poland are named after Esterka, usually ones associated with her and the King. In some sources Esterka is presented as King's consort who actually lived with him at Wawel Castle.
Seweryn Udziela Ethnographic Museum in Krakow. Krakowska street 46. 
- Byron L. Sherwin, "Sparks amidst the ashes", Oxford University Press US, 1997, pg. 125,