In Dragon Ball, during Gokou's first Tenka Ichi Bodukai, young Gokou had to fight Piccolo-Daimao (See Kame-sama for info on Piccolo-Daimao), the evil side of Kame-sama. Gokou killed him by head-butting right through him. However, before he died, Piccolo-Daimao spit an egg far into the mountians. This egg was created at the same time that Piccolo-Daimao died, and by some sick and twisted law of anime-phsyics, the egg replaced Piccolo-Daimao, thus not killing Kame-sama.

Many years later, at the next Tenka Ichi Bodukai, guess who Gokou met? Ma Junior, otherwise known as, our favorite Piccolo-san.

Born with the evil of Piccolo Daimo in his heart, Piccolo hated Gokou. For many years, Piccolo was the arch-nemesis of Son Gokou and the rest of the Z Senshi. But when Piccolo took it upon himself to train Gohan for the coming of the Saiyajins, a bond formed between them. By the time the Saiyajins arrived, Piccolo had lost much of the evil in his heart. The bond was so great, that Piccolo jumped in front of a blast from Nappa, saving Gohan from certain death. From then on, Piccolo was not only a powerful ally to the Z Senshi, but a good friend, too.

During the Freezer Saga, Piccolo met up with Nail, Piccolo's first encounter with another being from his race (other than Kame-sama). Since Nail was near death, Piccolo agreed to merge with him to give Piccolo a boost of power. And indeed it did, however, Piccolo lost much of the evil inside him, as well, because Nail (like the rest of the Namekseijin) was 100% pure-hearted goodness.

Later during the Cell Saga, Piccolo merged with Kame-sama, giving him an extreme boost of power that made him the strongest of the whole Z Senshi. With which he then proceeded to kick Cell ass.

Piccolo is around for the entire series, always there to help kick ass whenever needed. Piccolo dies for good in DBGT when Bebi destroys Earth, because Piccolo decides to stay with Earth becuase it is the only home he's ever known. And from there, he goes straight to Hell, where he stays.