Just as King Piccolo was dying from a gaping hole in the chest, he coughed up an egg and spat it into the sky. Son Goku, however, did not notice this.Piccolo Jr. is first seen as a little child roaming the woods after "hatching." He is evil from birth, and is jealous of a family he sees through the window. Even at this young age Piccolo Jr. possesses many of his father's, or past life's (depending on how you look at it) powers.
Years later, a teenage Piccolo makes as a return at the Tenkachi Budokai, planning on fighting Goku in it. In the finals, both Goku and Piccolo end up in battle. As they fight, Goku finds himself in the hardest battle he has ever been in up to that point, however, Goku still tries to play by tournament rules, which handicaps him. By the end of the fight, Goku defeats Piccolo, having managed to knock Piccolo out of the ring (and leaves him unconscious in the process). Goku does not kill him, and in fact wants him to be his rival, so that Goku would have the motivation to keep getting stronger.
Dragon Ball Z
After 5 years of training after his defeat to Goku, Piccolo notices a strong power on Earth, much more powerful than Goku. He goes to the source to find an alien named Raditz. Piccolo tries to defeat Raditz but fails. After Goku's son gets kidnapped by Raditz, Piccolo proposes that they work together, since alone they are both not match.
They go off to find the alien, and fight him. After awhile, it becomes apparent they are no match, so Piccolo tells Goku to hold Raditz off for 5 minutes, so he can charge his new attack, the Makankosappo (Special Beam Cannon). Piccolo fires the attack, but Raditz dodges last second. In desperation, Goku holds Raditz from behind, and Piccolo once again fires his Makankosappo, killing them both. Before Raditz dies, he announces that two stronger Saiyans will arrive on earth in one year.
To prepare for the next fight, Piccolo takes Goku's son Gohan to train him, at first this is his idea of revenge against Goku, but overtime he starts to grow attatched to Gohan, and helps him stay alive in the wilderness. After one year, Piccolo believes Gohan to have become stronger then him, but while battlingNappa, Gohan freezes in fear, and Piccolo blocks a Bomber DX from Nappa, in order to save Gohan, and, just before dying, said was his only friend.
Piccolo receives training from Kaio in the afterlife. He is eventually wished back to life and once Piccolo reaches Namek the home of his people, he fuses with Nail, a powerful Namekian warrior. Nail eventually fades away, shortly, this increases Piccolo's strength, dramatically rising his power level over 1 million. Piccolo defeats 2nd form Frieza, but is defeated by third form Frieza before being saved by his best friend Gohan. Piccolo, Gohan, Vegeta, and Krillin all try to damage Frieza's final form before Goku does it in Super Saiyan form.