Piccolo is the Namekian born to avenge his father who ends up as Gohan's mentor and one of the pillars of Goku's team. He appears in every series, from Dragon Ball to Daima, and goes from sworn enemy to the group's strategist; for his race, techniques and voice cast, see Piccolo's profile.
Dragon Ball
Piccolo Jr. Saga (episodes 123-153)
King Piccolo is the villain of the previous arc: in episode 122, run through by Goku, he manages to lay one last egg. From that egg, in episode 123, hatches the Piccolo we know, both his son and his reincarnation. In the Piccolo Jr. Saga he reaches adulthood in three years and enters the 23rd World Martial Arts Tournament under the alias Ma Junior, intent on killing Goku.
He beats Krillin and meets Shen in the semifinal, who is really Kami: Piccolo turns the Evil Containment Wave back on him and traps him in a small bottle. In the final, Goku tricks him into growing giant so he can free Kami, turns Piccolo's own techniques against him and, with injured arms and legs, sends him flying out of the ring in episode 148. Goku will not let Kami finish him off, since the death of one would kill the other, and even hands him a senzu bean; Piccolo leaves swearing he will win next time.
Dragon Ball Z
Saiyan Saga (episodes 1-35)
Five years later, in the Saiyan Saga, he teams up with Goku because Raditz stands in the way of his own plans for conquest. In episode 5 his new Special Beam Cannon drills through Raditz and Goku at once; talking to the dying Saiyan, he mentions the Dragon Balls, and the news reaches Vegeta and Nappa. Piccolo then takes Gohan into the wilderness to train him, cuts off his tail after his first Great Ape transformation and, without meaning to, grows attached to the boy.
When the Saiyans land, he learns that he is an alien from Namek. As Nappa fires at Gohan, Piccolo throws his own body in the way, and in episode 28 he dies telling the boy their time together mattered to him. Kami dies with him, and Earth's Dragon Balls stop working.
Frieza Saga (episodes 36-107)
In the Frieza Saga he trains in the afterlife with King Kai until Namek's Dragon Balls bring him back and transport him to the planet. In episode 77 he fuses with the dying warrior Nail, and in episode 80 he reaches the battlefield, where he holds his own against Frieza's second form. After Goku's Spirit Bomb, the tyrant blasts a hole through his chest; Piccolo wakes up on Earth and turns down a move to the Namekians' new home, choosing to stay close to Gohan and get stronger.
Garlic Jr. Saga (episodes 108-117)
In the anime-original Garlic Jr. Saga he pretends to be infected by the demon mist and, in episode 112, after attacking Gohan to sell the act, snatches the flasks holding Kami and Mr. Popo from Garlic Jr.
Androids Saga and Cell Saga (episodes 118-194)
In the Androids Saga his sharp hearing lets him learn who the mysterious Trunks really is. He trains for three years with Goku and Gohan, gets the better of Dr. Gero but cannot stop Android 17 and 18 from waking up, and they beat him. In the Cell Saga, in episode 141, he merges with Kami and becomes a single Namekian again; in episode 148 he fights Android 17 to a standstill, but Cell leaves him half dead and absorbs the android.
At the Cell Games he protests when Goku sends Gohan into the ring, takes on one of the Cell Juniors and, in the final clash, backs up his student's Kamehameha even though his help barely counts.
Majin Buu Saga (episodes 200-291)
In the Majin Buu Saga it is Gohan who invites him to the 25th World Martial Arts Tournament. In episode 215 he senses the power of the mysterious Shin, the Supreme Kai, and forfeits without fighting; in episode 221 Dabura turns him to stone with his spit. Once restored, he trains Goten and Trunks and, when Buu reaches Kami's Lookout, buys time by trapping him in the Hyperbolic Time Chamber. The monster breaks out anyway and in episode 265 absorbs him along with Gotenks to steal his intelligence too. Freed by Goku and Vegeta, he dies with the Earth, is revived and lends his energy to the Spirit Bomb. Ten years later, at the 28th tournament, he thinks he has not seen Goku this happy in a long time.
Dragon Ball GT
Baby Saga (episodes 17-40)
In the Baby Saga, in episode 26, he goes to recover Goten and is cut down by Gohan, now possessed by the parasite. In episode 40, with the Earth about to explode, he gives his energy to Goku for the last trip and chooses to stay behind: dying with the planet, he means to put the Black Star Dragon Balls out of use for good, and he says goodbye to Gohan telepathically.
Super 17 Saga (episodes 41-47)
In the Super 17 Saga he gets himself sent to Hell on purpose, where Goku is trapped. In episode 45 he and Dende open a portal to send Goku back to Earth, but Piccolo has to stay perfectly still to keep it open, so he remains below. In the series finale Goku stops by to say goodbye and promises he will get out one day.
Dragon Ball Super
Resurrection 'F' Saga (episodes 19-27)
In the Resurrection 'F' Saga he takes on Frieza's army, restarts Gohan's heart and, at the end of episode 22, steps in front of his student to shield him from Frieza. In episode 23 he is dead and Goku carries his body to the Lookout; revived with Namek's Dragon Balls, he agrees to train Gohan again.
Universe 6 Saga and Future Trunks Saga (episodes 28-76)
In the Universe 6 Saga, in episode 34, he introduces himself as Universe 7's strategist: with illusory copies and stretching arms he pins Frost down, then suddenly passes out, just as Goku had. Frost is disqualified for cheating and Piccolo gives his place to Vegeta. In the Future Trunks Saga he teaches Trunks a technique to use against the enemy.
Universe Survival Saga (episodes 77-131)
In the Universe Survival Saga he whips Gohan back into shape in episode 88. In the Tournament of Power he often fights by his side: in episode 119 he stretches his arms to save him from falling, eliminates a Universe 4 fighter and is then knocked out by an invisible enemy he cannot land a hit on.
Dragon Ball Daima
The journey through the Demon Realm (episodes 1-20)
As our Daima episode guide explains, Piccolo is shrunk along with the others by Gomah's wish and reaches the Demon Realm with Vegeta and Bulma. In episode 11 he sees through old Neva's senile act, but finds he cannot speak the Namekian language needed for the wishes. In episode 16 he takes Degesu down so Panzy can rescue Dende; in episode 20 he strikes Gomah twice on the back of the head, one blow short of knocking out the Third Eye.
Movies
In Dragon Ball Super: Super Hero Piccolo trains Pan, survives an ambush by Gamma 2 and infiltrates the new Red Ribbon Army. A wish to Shenron unlocks his potential and adds the Orange Piccolo form; he fights Cell Max giant to giant and holds him for Gohan's final blow. In Dragon Ball Super: Broly he is the one who helps Goku and Vegeta pull off the Fusion, on the third try.
Our take
Read saga by saga, Piccolo's story is a slow shedding of skin. He is born as a weapon of revenge, becomes an ally of convenience against Raditz, and the point of no return is Nappa: a demon dying for a child. From then on every choice he makes runs through Gohan, up to the repeat sacrifice in Resurrection 'F' and the training sessions with little Pan in Super Hero.
He is also the character who most often gives up the spotlight: he forfeits against the Supreme Kai, hands his match to Vegeta against Frost, stays in Hell in GT. Toriyama built Super Hero around his bond with Gohan, and that film feels like the reward for decades spent as the sidekick.
Did you know? After the merge with Kami, Goku jokingly suggests a name that combines the two ("Kamiccolo"), but the Namekian prefers to keep calling himself Piccolo.
For the other half of this bond, read Gohan saga by saga; for his lifelong rival, see Goku's profile.