Piccolo was the "bad" half of the Guardian of Earth known as Kami. To become Guardian of the Earth Kami had to rid himself of the evil Piccolo and Piccolo became a seperate being. As a Namek born on the planet Namek Piccolo is a strong warrior. At first he was evil and thrived on destroying Goku but Piccolo in the early DBZ showed that he was no longer evil at all. As a namek he has mastered many moves such as regeneration and the special Beam Cannon and body extension and many more. He has proved a worthy foe against all oppenents however Piccolo wasnt ever stronger than a Super Sayian except once. Still Piccolo held a huge role in DB/Z. He fused with the Namek Nail and became much stronger. Later in DBZ he fused with Kami again and became incredibly strong! Overall Piccolo is the strongest of the Z fighters who is not a saiyan. Piccolo is a very smart fighter as well. He always knows what attack to use at the right time.(unless he is angry then he sometimes does something stupid). The last thing to be said about Piccolo is that he is always up to a challenge. No matter how hard the odds are he keeps on going. Unless hes unconious or something.
