Hiruzen Sarutobi

The Third Hokage is a former student of the First Hokage and Second Hokage. Because of his surname Sarutobi the First and Second called him Saru. He is the father of Asuma Sarutobi and the grandfather of Konohamaru. The Third was called The Professor due to his vast knowledge of jutsu purportedly knowing all the jutsu within Konoha and also as the God of Shinobi due to his immense power. Information taken from Wikipedia.