Piccolo, in my opinion, is one of the most interesting characters in the Dragon Ball Universe. Let's review his history and see how much he's changed.
Guardian of the Earth:
I've mentioned this elsewhere, but it seems that in the original Dragon Ball, Kami was meant to be God and Piccolo the Devil. This is shown to be true by the kanji on their robes. Initially, Kami desired to become the new guardian. However, the current guardian stated that Kami had too much evil in his heart. If Kami were to become guardian, he was need to cast that aside. This resulted in the birth of Piccolo.
King Piccolo:
The evil half of Kami was very powerful and at one point, was known as the Evil King Piccolo. He was hellbent on ruling over the Earth and created his children, which were dinosaur-like creatures, to help him achieve this. This is one of the many continuity flaws in Dragon Ball that never gets explained. We know that Nameks are all male and reproduce asexually. However, that should produce more Nameks. What King Piccolo created, was much different and seemed to elude to some type of demon monsters in his status as the Devil at the time.
Piccolo Jr.:
The last battle between Goku and King Piccolo lead to an epic moment where Goku, nearly defeated, blasted through King Piccolo by propelling his body via a one-handed Kamehameha and drilling right through his opponent's mid section. In his dying moments, Piccolo created one final egg in which he transferred is very essence into. This was the birth of Piccolo Jr., who Goku would later battle as a teenager in the World Martial Arts tournament.
Piccolo:
Years later, we see Goku as a married adult with a child, Gohan. Piccolo had been lying low after being defeated in the World Martial Arts tournament, but still had great hate for Goku. However, the Dragon Ball series took a huge turn, introducing the Saiyans. At this point, Goku and Piccolo decided to work together to take down Raditz, leading to Goku's death. Knowing that backup was coming, Piccolo decided to take Gohan under his wing and train him. Gohan and Piccolo eventually bonded, turning Piccolo into one of the good guys, eventually throwing himself in harm's way in order to protect Gohan. He still didn't quite like Goku at this time, but that was all about to change.
Piccolo - Sync with Nail:
We eventually learn that Piccolo and Kami are not from Earth, but a planet called Namek. Bulma, Krillin, and Gohan traveled to the planet in order to ask the Nameks to use their Dragon Balls, which would allow for the fallen Z fighters to be revived. After many struggles, Piccolo arrives on the scene only to find his Namek brother, Nail, almost dead at the hands of Frieza. Nail makes an offer to Piccolo: they will join bodies to become an even stronger Namek. Piccolo eventually agrees after Nail tells him he will remain his own being. Piccolo becomes a strong opponent, and is able to keep up with Frieza in his base form.
Piccolo - Fuse with Kami:
During the Cell saga, the Z fighters are no match for the androids raging war on them. Kami and Piccolo greatly despise one another, but knowing what's at stake, they reluctantly decide to fuse and become one being once more, much to the dismay of Kami's long-time friend and assistant, Mr. Popo. This fusion left Piccolo the sole Namek while inheriting great power. He become stronger than all the Super Saiyans and really kicked some ass.
Piccolo - Mentor, Ally, Friend:
Through the rest of the series, Piccolo seems to have taken a back seat to actually fighting, becoming more of a mentor. He still helped out where he could, but as the Saiyans grew more powerful, he and the other Z fighters were quickly over shadowed. In the Buu saga, Piccolo was a main character in that he was responsible for training Goten and Trunks the Fusion technique. Piccolo does return to action in Dragon Ball Super, fighting in an epic battle between universes.
So this is basically all from memory, but feel free to chime in. There's no doubt that Piccolo has become one of the most evolved characters in Dragon Ball and has had great influence on the series.
Guardian of the Earth:
I've mentioned this elsewhere, but it seems that in the original Dragon Ball, Kami was meant to be God and Piccolo the Devil. This is shown to be true by the kanji on their robes. Initially, Kami desired to become the new guardian. However, the current guardian stated that Kami had too much evil in his heart. If Kami were to become guardian, he was need to cast that aside. This resulted in the birth of Piccolo.
King Piccolo:
The evil half of Kami was very powerful and at one point, was known as the Evil King Piccolo. He was hellbent on ruling over the Earth and created his children, which were dinosaur-like creatures, to help him achieve this. This is one of the many continuity flaws in Dragon Ball that never gets explained. We know that Nameks are all male and reproduce asexually. However, that should produce more Nameks. What King Piccolo created, was much different and seemed to elude to some type of demon monsters in his status as the Devil at the time.
Piccolo Jr.:
The last battle between Goku and King Piccolo lead to an epic moment where Goku, nearly defeated, blasted through King Piccolo by propelling his body via a one-handed Kamehameha and drilling right through his opponent's mid section. In his dying moments, Piccolo created one final egg in which he transferred is very essence into. This was the birth of Piccolo Jr., who Goku would later battle as a teenager in the World Martial Arts tournament.
Piccolo:
Years later, we see Goku as a married adult with a child, Gohan. Piccolo had been lying low after being defeated in the World Martial Arts tournament, but still had great hate for Goku. However, the Dragon Ball series took a huge turn, introducing the Saiyans. At this point, Goku and Piccolo decided to work together to take down Raditz, leading to Goku's death. Knowing that backup was coming, Piccolo decided to take Gohan under his wing and train him. Gohan and Piccolo eventually bonded, turning Piccolo into one of the good guys, eventually throwing himself in harm's way in order to protect Gohan. He still didn't quite like Goku at this time, but that was all about to change.
Piccolo - Sync with Nail:
We eventually learn that Piccolo and Kami are not from Earth, but a planet called Namek. Bulma, Krillin, and Gohan traveled to the planet in order to ask the Nameks to use their Dragon Balls, which would allow for the fallen Z fighters to be revived. After many struggles, Piccolo arrives on the scene only to find his Namek brother, Nail, almost dead at the hands of Frieza. Nail makes an offer to Piccolo: they will join bodies to become an even stronger Namek. Piccolo eventually agrees after Nail tells him he will remain his own being. Piccolo becomes a strong opponent, and is able to keep up with Frieza in his base form.
Piccolo - Fuse with Kami:
During the Cell saga, the Z fighters are no match for the androids raging war on them. Kami and Piccolo greatly despise one another, but knowing what's at stake, they reluctantly decide to fuse and become one being once more, much to the dismay of Kami's long-time friend and assistant, Mr. Popo. This fusion left Piccolo the sole Namek while inheriting great power. He become stronger than all the Super Saiyans and really kicked some ass.
Piccolo - Mentor, Ally, Friend:
Through the rest of the series, Piccolo seems to have taken a back seat to actually fighting, becoming more of a mentor. He still helped out where he could, but as the Saiyans grew more powerful, he and the other Z fighters were quickly over shadowed. In the Buu saga, Piccolo was a main character in that he was responsible for training Goten and Trunks the Fusion technique. Piccolo does return to action in Dragon Ball Super, fighting in an epic battle between universes.
So this is basically all from memory, but feel free to chime in. There's no doubt that Piccolo has become one of the most evolved characters in Dragon Ball and has had great influence on the series.