Personality

Piccolo is the most relevant Namek in DragonBall (Z). Like every Namek, he has green skin and antennas. He wears a good-looking outfit, which consists in a turbant, with a purple area, a cape, partially folded over his shoulders, and a plain bluish purple suit beneath it. He is rather tall, too. Why should I be describing him, anyway, if you have a picture on the left?
He has the capability of absorbing other Nameks, becoming much stronger. For instance, before fighting Freeza, he merged with Nail, an injured Namek, and later, after Cell appeared, he absorbed Kame-Sama, Earth's god and Piccolo's complement. If one dies, so does the other and the DragonBalls won't work any more. With this last absorption, Piccolo becomes very strong and, to replace Kame-Sama for a full-time god, Dende is picked up by Gokuh on the Nameks' new planet.
In DragonBall, he was a major bad guy, but in DBZ he proves that he has a good heart. He is particularly admired by Son Gohan, whom he brought up for a while.

Technique

Piccolo's favourite way of improving his skills is probably by meditation. He's not the social type, so silent, smooth mental training does wonders.
His most radical power increases were by absorbing other Nameks (Nail and Kame-Sama, especially the latter). His powers never seem to be enough to beat the worst opponents in the final combats, so all these power increases may feel a bit frustrating (personal opinion). Anyway, I feel Piccolo is always the character that stands back, ready to support the other fighters with his great knowledge and experience, instead of going into actual battle. He does this perfectly and his psychic powers help him to have a clear picture of the situation, spotting small details that often are ignored by the unexperienced, young or careless eye of some of the other, more hands-on characters.
His most famous technique is the Moganzozanpo, which consists in a powerful energy drill shot from Piccolo's finger after a moment of concentration. It is fast and dangerous (to his opponent, of course). It is not very "thick" in terms of the released energy wave, and that make it a precise, elegant attack, unlike the KameHameHa that just blows everything up in its path. Other of Piccolo's techniques is the Ghost Fireball, a damaging strike that is quite hard to block or dodge.

Piccolo, who else?

Roles in the major battles

Battle against:	Role:	A DBZ card with Piccolo on it
Vejita	Piccolo helped to beat Nappa during this confrontation and saved Son Gohan from certain death by placing himself between his pupil and one of the foes' energy ball. He died, but was resurrected afterwards.
Freeza	Like Vejita after him, he seemed to be a valid hope to beat Freeza. He had absorbed Nail before entering the combat site and felt very powerful. Not powerful enough to beat Freeza, though. Some time later, he was seriously injured by Freeza and it was Gohan that took him away to the space ship.
Cell	To fight Cell, Piccolo merged with Kame-Sama, which increased dramatically his skills. He could have beaten Cell in his 2nd. evolutionary state, but he got away after using the sun light to temporarily blind the near-by fighters. Cell wasn't seen again until he found #17, who was fighting Piccolo, and absorbed him. Later, he absorbed #18 too and Piccolo was now no match for Cell.
Buu	In Portugal, we haven't finished watching the series, so I can't yet fill in this section, sorry.