Dragon Ball Z is undeniably one of the most beloved anime sagas of all time. Planet-splitting action, intense fights -- the reasons for its popularity are endless. It's undeniably popular. Therefore, it's more than a little odd that its main protagonist, Son Goku, is seldom referenced as anyone's favorite character. Sure, he is the most powerful and has some of the most badass scenes in the series, but he isn't as popular as Vegeta, Piccolo, Trunks or even many of the villains like Frieza, Cell or Broly.
Why is that?
The simple truth is this: Goku isn't really a central character in Dragon Ball Z. He isn't the main character. Very little in this series happens because of Goku. But if Goku isn't the main character, who is? Well, the truth is... nobody.
What is a Protagonist?
It may seem silly, but if we're gonna dethrone Goku as the protagonist of Dragon Ball Z, we have to briefly establish what a protagonist is.
The protagonist of a series is its central character. Decisions that direct the flow of the plot are made by them. Their actions influence and impact the path of the story. While a writer can create a passive protagonist, many readers usually identify or gravitate toward characters whose actions drive the plot.
Protagonists also usually undergo an arc where they change and develop as a character. The change can be for better or worse, as many protagonists actually become worse people as the story goes on (such as Macbeth or Light Yagami). This isn't always the case, though, as many more protagonists overcome personal shortcomings as a result of their journey.
So how does this play into Dragon Ball Z?
Goku Already Completed his Arc
Goku is undeniably the protagonist of Dragon Ball, but something changes once Dragon Ball Z starts.
In Dragon Ball, Goku is the central focus. He goes on a journey with Bulma to collect the Dragon Balls. Then, he trains with Roshi and Krillin. He fights the Red Ribbon Army. It is his emotional journey audiences are taken on during the Piccolo arc, with him having to process his emotions following Krillin's demise and his master being defeated, with him carrying through the legacy of his master and Kami in the process of beating Piccolo.
The story is his coming of age, from childhood to adulthood.
The only thing is, he really doesn't develop much after that. Coming out of Dragon Ball, Goku is a fully-formed character. He doesn't undergo any real change between learning of his Saiyan heritage and flying off with Uub at the end of the series. He grows stronger, sure, but he still makes goofy decisions, loves battle and maintains a pretty consistent desire to prove he is stronger than anyone else. His only decisions allow other characters to take center stage (such as encouraging Gohan to fight Cell, staying dead and not returning following the battle against Frieza on Namek).
Furthermore, in terms of screen time, Goku is barely there. He spends most of Z either training, dead, traveling or waiting to train. He fights a few times, but these fights barely influence the plot. His fights with Android 19, Cell, Yakon, Majin Vegeta, Buu as a Super Saiyan 3... what do they really contribute to the story? Half of his fights don't change the narrative.
All of these decisions Goku makes, by the way, put Gohan in a position where he becomes the protagonist of the story.
Legacy of Gohan
It is clear with Gohan's introduction and Goku's death during the battle with Raditz that Akira Toriyama wanted Gohan to take center stage. This first arc is, in essence, the transitional period. Goku is dead and training, while Gohan is being trained by Piccolo. In many respects, Gohan's arc throughout the Saiyan Arc to Cell Arc runs parallel to Goku's arc in the original series. He is trained, follows through the legacy of his masters before conquering a green-colored adversary who, like Piccolo, is tied to a past conflict.
Even still, his arc in the Buu Saga is mostly directed by his actions. He grows as an adult -- well, not in terms of strength since he really doesn't train all that much, but his actions, for the most part, direct the flow of the part, right up to when Buu absorbs him.
The problem is that Gohan really only has moments where he is the main character. The final fight with Cell really is where he transitions from a supporting character to a main character. Before that, however...
Piccolo and Vegeta
Both Piccolo and Vegeta influence the plot far more throughout the series than Gohan or Goku. Piccolo's growth from villain to father figure is one of the most distinct arcs in the series. Indeed, for most of the Saiyan and Android Saga, Piccolo's decisions influence the flow of the plot.
Vegeta, on the other hand, is basically the protagonist of the Namek Arc. His decisions impact the story. His perspective informs audiences of the legend of the Super Saiyan, which becomes incredibly relevant during Goku and Frieza's arcs. During the Cell Arc and Buu Arcs, his decisions allow the antagonists to end up emerging on top. It's Vegeta who comes up with the idea of the Spirit Bomb that ultimately defeats Kid Buu.
Vegeta and Piccolo grow up, develop families, learn to love. They have more development and screen time than either Goku or Gohan.
The Ensemble Cast
But the real answer is this: DBZ doesn't follow one protagonist. It follows a set of different characters for periods of time, juggling from one character to the next. No one character throughout the series remains in the spotlight for more than their part.
Dragon Ball Z is a great example of an ensemble cast. Each character has an impact. Each character contributes. But, on the whole, no one character is the protagonist.
So, while Goku is often mislabeled as the protagonist, this is only because he was the protagonist in Dragon Ball. But Dragon Ball was one character's journey. DBZ is a far more epic story, and, thus, requires a broader lens to tell the full story.