I won't profess to be the greatest of Dragon Ball Z fans; in fact, that's more my brother's area of expertise than mine. But out of all the characters on the show, Piccolo is my favorite. He's the one that seems the most in depth of characters; he's not bad, but nor is he good. Piccolo is more of an antihero, one that falls somewhere in between.
His full name is Piccolo Daimao; he is a Namek Saijin from the planet Namek. Piccolo is the offspring of Daimon, the Evil that seperated from Kami-sama and lived on in its own body. The two - Piccolo and Kami-sama are bound forever this way, since they are one being in two bodies. When one dies, so shall the other. When Son Goku killed him, Daimon swore that his offspring would exact vengeance. Thus, Piccolo wants Goku dead.
After finding himself in an unlikely alliance with Goku to defeat the warrior, Raditz, Piccolo was finally able to kill him. Goku sacrificed himself when Piccolo used the Mankankosappo, an incredibly powerful attack, and died with Raditz. The victory was short-lived however, for Raditz warned the Namek that others would come: two other Saiyajin, more powerful than he, would come to avenge his death. It was then that Piccolo took Goku's son, Gohan under his wing and began to train him to be a warrior to fight against the Saiyajin.
Gohan is very dear to Piccolo, though he rarely shows it. His training strengthened and brought his tremendous talent to blossom. Piccolo raised Gohan like a son, and his love for the boy was acknowledged when the Namek sacrificed himself for the sake of Gohan in the battle against Nappa.