Piccolo was once and arch enemy to almost everyone back in Dragonball and early in Dragonball-Z, but after seeing Goku lying dead on the ground, his mind softened and decided that that was not what he wanted. In the Dragonball series, he fought Goku numerously, but lost every time. Finally, Goku destroyed King Piccolo, but before King Piccolo dies, he generates his power into an egg and shoots it onto a mountainside where it hatches and becomes the Piccolo we know today. With the help of Goku in the Dragonball-Z series, they destroy Raditz. Then he trains with young Gohan for a year so he could fight the Saiyans. He dies heroically by standing in front of a blast meant for Gohan, his best friend at the time. He then trains with King Kai and is soon wished back on Namek to fight Frieza. On his way to fight the evil Frieza, he fuses with Nail, a strong Namekian Warrior. He fights Frieza doing good, but when Frieza transforms, he gets his butt whipped. He is revived by Dende, another Namekian. After Frieza was hit by Goku's Spirit Bomb, he heroically gets hit by a beam meant for Goku. He is revived and doesn't do much more in the Frieza Saga.
In the Cell Games, he fights the androids and cell doing pretty well after he fuses with Kami. When Cell becomes Perfect he doesn't really do much else.
