As the last antagonist of the previous series, Piccolo becomes one of the major Z Fighters by the time of Dragon Ball Z. He is by far one of the most popular characters in the entire series, though probably just a little under Vegeta.
Since most of us American kids actually watched Dragon Ball Z before Dragon Ball, we really did not know the animosity between Goku and Piccolo. But with Goku being in the afterlife, Piccolo was technically the strongest fighter defending the Earth. He claimed he only wanted to protect the Earth so he could conquer it later, but due to the love he had for Gohan, that turned out to be far from the truth. We also have that legendary moment where he sacrifices his life for Gohan against Nappa.
He spends the majority of the time during the Namek Saga on King Kai’s planet, training with the rest of the fighters who fell against Nappa. All of them eventually leave the planet to become stronger, leaving the dead Saiyan who put them there. Piccolo manages to send a message to have himself wished back to life and is transported to Namek.
He luckily runs into Nail, who is dying anyway, but Nail manages to teach him a fusion technique, allowing Piccolo to absorb a lot of power while retaining no real traits of Nail’s personality, especially if you discount the English dub. He then has a pretty cool fight against Frieza when the emperor of the universe decides to reveal his gigantic, terrifying form.
Frieza keeps powering up more and more, and by the end of the battle, Piccolo has taken quite a beating, despite being the fourth strongest fighter in the universe at the time.
He briefly becomes the strongest character in the series after fusing with Kami to restore his original self. He immediately tries to kill the Androids before Cell can absorb them, but his plans go completely wrong, and he basically becomes irrelevant as a fighter afterward.
During the Majin Buu Saga, he gives up a sparring match against Supreme Kai because he fears his power. Somehow, he becomes the babysitter and mentor of Goten and Trunks, and thanks to him, that plan almost works.
I was actually really surprised to see Majin Buu absorb him at the same time as Gotenks, but apparently Buu wanted more of Piccolo’s mind than his power. Goku and Vegeta eventually manage to rescue him from Super Buu’s body, and he then immediately dies when Kid Buu blows up the Earth. Most characters did not have a good time during the Majin Buu Saga, but Piccolo might have had it the worst, especially considering he was turned to stone by Dabura and broken into pieces by the kids.