Rabbi Levi Yitzchak is one of the most popular and beloved figures in Jewish history. Born into a prominent rabbinical family, he studied with the famed author of the "Pri Megadim," was himself a great scholar and served as rabbi of a number of Polish communities. However, after meeting Rabbis Shmelke Nikolsburg and Dov Baer of Mezhirech, he was won over to Chassidism and eventually became one of the most influential leaders of Chassidism in Central Poland and the Ukraine. He is particularly famous for defending the Jewish people before G-d and always interpreting their actions in the best possible light. His fervor in prayer and the fulfillment of Mitzvot are legendary. His song before Havdalah, "Dudele", expressing mans yearning and awareness of G-ds presence, is still sung with tenderness and great feeling. Rabbi Levi Yitzchaks work, "Kedushas Levi," was published in his lifetime and is ever popular.
Sources: Orthodox Union