R' Yaacov Yosef of Polnoye (1705-1784), was one of the first and most dedicated of the disciples of the founder of Chassidus, the Holy Baal Shem Tov. He was already an accomplished scholar when he attached himself to the Baal Shem Tov, and his becoming a disciple engendered much controversy, and he was even booted of of the the city of Sharograd where he had been the Rabbi for a number of years.
His book Toldos Yaacov Yosef (1780) was the first chassidic work ever published. In it repeats the phrase, "I have heard from my teacher", 249 times. He is one of our foremost sources for teachings from the Baal Shem Tov.
Reb Yaacov Yosef was as well a Cohen and somewhat well known for his abrupt temperment.