First introduced in "Dragon Ball" and the unequalled hero of the saga, Goku is a powerful figure from the very start. In the course of the Saiyan Saga, we learn more about Goku than was ever revealed in the saga of his younger days. From the start, we meet his new family and the lifestyle he has settled into as he became an adult. The laid back life he has built for himself and his family is shattered almost immediately when the evil Radditz arrives on Earth and kidnaps Goku's son. This triggers the start of an epic saga in which Goku becomes more powerful than anyone could ever imagine, especially his friends.
Almost immediately removed from the core action of the show and sent on a journey to become stronger so he can save the Earth, Goku always strives to be better. We are told from the very beginning of this series that he is one of the last members of an alien race and was originally intended to conquer the Earth, not assimilate and become one of the humans there.
While Goku's love for his family and friends is unequaled, he is often removed from them. Goku's focus and his primary goal throughout "Dragon Ball Kai" is to grow stronger.
As Goku becomes more powerful, his entourage's role in the story becomes less important as a physical presence and takes on the role of chorus – the watchers who comment on the fight, ponder the implications of his actions, and pray for his safety. The Earth is in Goku's hands numerous times in just the first handful of episodes and as the stakes quickly escalate, so too do the expectations of that chorus.
Goku's story is frequently one of sacrifice, putting every ounce of his energy and physical being into fighting an all powerful foe. The threat of death is very real for Goku in every moment of his fight with Vegeta and Nappa. This hangs over Goku's head for the remainder of the series and makes his presence that much more tenuous.
Named after Goku's grandfather, Gohan is first introduced in Dragon Ball Kai as an innocent young boy who is frightened by strangers and yet has the same curiosity about the world around him as his father once did. The impact and presence of his mother, Chi-Chi, is felt in those early interactions as Gohan is a quieter version of his father – one who doesn't always know where he stands in that world.
Gohan's innocence is quickly shattered when Radditz arrives and kidnaps him. Thrust into the center of a fight he does not understand and revealed to be the descendant of a race even his father didn't know of, his innate nature starts to reveal itself in short, unexpected bursts of power. From his first lashing out against Radditz to the brutal training Piccolo puts him through, Gohan is tested frequently through the first episodes of the show and he changes dramatically because of it.
While Gohan learns to be independent and "grows up" in the course of only three or four episodes, he remains a young child. We see glimpses of what Gohan is capable of, including an important showdown with Piccolo after seeing his first full moon. Yet, Gohan is still unable to handle the pressure of the situation into which he has been thrust. His power is there and it grows substantially over the course of just a few months (and once the Saiyans arrive only a few hours), but the naivety of a child shines through when it matters most.
Gohan's gentle nature, undercut by bursts of rage, is a call back to the split nature of Goku's Saiyan background and human upbringing. The duality of Goku's personality is shown with the half-breed Gohan, and yet the potential is so much greater for him. The main question we are left to ask at the end of this saga is not whether Gohan can become as powerful as his father, but whether he wants to do so.
At times pushed to the chorus of supporting players during the fight with the Saiyans, there are glimpses of greatness in Gohan's actions. Within the context of this saga, it remains unclear whether that will be the case.
First introduced as the premier member of the Saiyan race – a prince in exile who makes his living by conquering and at times destroying entire races of people – Vegeta is a terrifying opponent for Goku. Upon the revelation of Goku's origins, we are almost immediately introduced to incredibly powerful beings who have been operating at stakes far beyond anything Goku and his friends have ever considered.
We are given few glances into Vegeta's history in the Saiyan saga. Rather, we are presented with a brutal tyrant who is appalled at what Goku has become. His contempt quickly turns to a mix of jealousy and fury when he realizes the power and ability of his opponent. Having never been challenged in the way that Goku challenges Vegeta, he grows angry and the pleasure of finding a worthwhile opponent quickly evaporates.
He has long expected and has been the most powerful Saiyan in the universe and among the most powerful fighters. When he stumbles upon a lost, tailless Saiyan who has fallen in love with a human woman and started a family instead of conquering the very planet on which he was raised, incredulity turns to anger quickly. It becomes evident that he needs to do something different to achieve his true potential.
This becomes a recurring theme of the show. Goku and Vegeta are reflections of each other, raised in different circumstances. Goku's obliviousness and general compassion for those around him is non-existent in Vegeta, who is a cold, selfish, and at times brutal adversary. He abhors anyone who shows weakness of any sort, going so far as to destroy his own allies when they show themselves unable to compete at the level he expects.
He seeks perfection in his form and in his role as the ambassador of the Saiyan race. He is what Goku could have and should have been – fully aware of his capabilities and having spent a lifetime practicing his trade and becoming an expert in the martial arts. This expertise, however, is matched by Goku who has done none of those things, triggering the start of a prolonged and at times bloody rivalry between the two saiyans.
Krillin is the most powerful human fighter on earth and still pales in ability to the Saiyans. In the original Dragon Ball series, Krillin's role was as archetypal beta to the alpha level abilities of Goku, and in Dragon Ball Kai that continues, though Krillin plays a more central role in some events.
He remains as one of only three earth fighters before Goku's arrival (and the only remaining human). Despite that strength, Krillin's diminutive size, monk-like nature, and frequent role as "leader of the others" denotes him as a part of the chorus that frequently gives the viewer insight into the fights between the Saiyan protagonists and their opponents.
While his role is frequently reduced to comic relief, Krillin acts as a secondary surrogate and support for Gohan during the fight with the Saiyans, filling in for Goku as much as possible while the boy's father finishes his training and returns to Earth.
As an immediate twist in the first episode of the series, Piccolo becomes an ally to the Z fighters. As the culmination of Goku's challenge in Dragon Ball, he is almost immediately surpassed as a threat by the arrivals of Radditz, Nappa, and Vegeta, and is forced to join forces with his former enemies to combat them. The threat to the Earth (and his own ability to conquer it) make it necessary for him to join Goku.
It takes little time for Piccolo to become an integral part of the Z Fighter team and the primary mentor and teacher for Gohan who has, as of yet, never been tutored by Goku in the use of his powers. By fostering Gohan in the time Goku is away training in the other world, he becomes a vital link between the strange, alien power that resides in Gohan and the human world he seeks to protect.
The time spent with Gohan changes Piccolo nearly as much as it does the young half-Saiyan, building in him a sense of pride in his training and a genuine care for Gohan. The connection between Piccolo and Kami becomes a vital thread of the story during the fight with the Saiyans.
We are introduced to King Kai early in Goku's training in Other World. He is the Guardian of the North quadrant of the universe and is therefore able to see and interact with all that happens in those realms. This gives him a god-like presence in the Dragonball Z Kai story but a sort of distance from the actual fighting as he doesn't personally have the strength needed to combat those threats.
He is a spiritual mentor to Goku and the one who begins to unlock the true potential of the Saiyan's innate talents. He also represents comic relief during a period of tension in the story. When Goku is trying increase his strength so he can return to the world he loves and protect those he cares about, King Kai keeps the fighter grounded and focused through frequent jokes and games. The strange ways of this eccentric old deity are vital to Goku's training, though, and ultimately allow him to utilize new techniques and unlock untold powers that are enough to take on Vegeta.
Radditz is an important element of the Dragonball Z Kai story, even if he only appears for three episodes. It is revealed almost immediately that Radditz is Goku's brother and that both of them are Saiyans – members of an alien race from another planet. When he first arrives and calls Goku by his true name (Kakarot), he is the ultimate disruption to the status quo and the gentle, domestic life that Goku has built for himself.
This interruption to the happily-ever after environment of Goku's world is the trigger that kicks off the entire story, from bringing Nappa and Vegeta to Earth to the ultimate showdown on Namek. Radditz faces Goku and Piccolo in a fight that shows just how much more powerful this otherworldly threat will be. He is a pure representation of all that we do not know, and how the road to power continues far beyond what we can imagine. Goku's travels will never truly be finished.
Nappa is a massive, physically imposing being that represents everything one expects to see in a creature of rage and power. He is the Goliath of the Dragonball Z universe and is ultimately pitted against a number of Davids who almost bring him down. He is the foil to Vegeta, whose cool and calculating form of destruction is far more powerful and hides a much deeper well of potential than Nappa's muscular frame.
The first stages of the fight with the Z fighters are almost entirely fought against Nappa, creating a sense of dread as Vegeta's snaps commands and dictates the terms of the battle. This disparity is used to highlight just how much more powerful the top tier fighters in this universe are and how ultimately unimportant the physical trappings of strength end up being when Goku and Vegeta enter the fight.