King Piccolo is the father of Piccolo Jr. King Piccolo came from a Namekian who divided himself into two after being rejected for the position of the next Guardian of the Earth. The two entities from that Namekian was Piccolo and Kami, the good counterpart of Piccolo.
Piccolo brought war to the Earth which caused many casualties, with the help of his many sons. He is intelligent but very aggressive and merciless, willing to do anything to get what he wants. He also believes that his power is the best among the rest.
This is until King Piccolo was defeated by Goku during the second time that they battled. Goku ended up killing King Piccolo. King Piccolo then he decided to take revenge on Goku by spitting up his final son, Piccolo Jr.