Namek as the “Kaio of the North Galaxy,” and later in chapter 328 he tells Bulma that Namek is outside of his “domain” (apparently meaning the North Galaxy). The Dragon Ball Z Anime Special II, a magazine released shortly after chapter 328, features a roundtable interview with Toriyama and several DBZ anime staff members, including series organizer Takao Koyama and chief animator Minoru Maeda. Koyama mentions that since the Kaio we know is actually just the Kaio of the North Galaxy, then “that means there must be a South, East, and West Galaxy, as well…” Maeda says “they could be quadruplets,” and Toriyama likes the idea: “Quadruplets! Oh, that’s good.”
Since Kaio sits out most of the Cell story arc, it’s not until the Buu arc that Toriyama gets around to incorporating the idea that the blue catfish is just one in a set of four into the main story. In chapter 428, the South Kaio turns up to watch as Goku trains under North Kaio’s supervision. He’s curious to see Goku since Goku’s supposed to be one of the best warriors in the “North Area” (for whatever reason, the term “galaxy” isn’t used here and the two Kaios’ domains are referred to as the “South Area” and “North Area”). Shortly afterwards, the DBZ anime aired the afterlife tournament, a filler story arc set between the Cell and Buu arcs, and featuring all of the Kaio. Not only do we meet the Kaio of the North, South, East, and West Galaxies (it’s “galaxies” again, instead of “areas”), but we even get to see their boss, the Grand Kaio, the god in charge of the entire universe. Toriyama provided the character design for the Grand Kaio and also the distinctive “snow globe” style design for the Dragon Ball cosmos, briefly shown during DBZ episode 195. It’s safe to say that Toriyama’s involvement with this filler arc was probably a factor in him introducing South Kaio into the manga a little while before he and the other extra Kaio popped up in the anime.
Besides Maeda’s playful suggestion of Kaio quadruplets, this new setup probably takes some inspiration from the Four Heavenly Kings of Buddhism, a group of gods who watch over the four cardinal directions. They turn up in Journey to the West, and in Japanese pop culture any group of four outstanding individuals tends to get labeled as “Four Heavenly Kings” in reference to them. That’s where the Elite Four in Pokemon get their Japanese name from, and elsewhere in Dragon Ball Garlic Junior turns up with his “Four Heavenly Kings of the Demon Clan” (aka the “Spice Boys” in FUNimation’s English dub). Though it’s more of a stretch, we might also remember how Dorothy and friends made a cameo at the Tenka’ichi Budokai, and that the Land of Oz is likewise divided into four regions (north, south, east, west), each ruled by a witch. Another inspiration? Actually, Toriyama seems to have a thing for dividing stuff up based on the cardinal directions. Besides splitting the Dragon Ball universe up into four galaxies/areas, the main locations on DB Earth follow the same pattern: West City, South City, East City, North City…as above, so below.
Grand Kaio’s reign as top dog is short-lived though. In chapter 440, Piccolo asks this mysterious “Shin” fellow if he’s the Grand Kaio (the first and only time he’s mentioned in the manga), but he turns out to be someone even higher: the Kaioshin, literally the “God of the Kaio.” So not only do you have a god on each planet (like the God of Earth), but you’ve also got kings of those gods (the Kaio), and those god-kings have a god of their own (Kaioshin). And of course, Kaioshin isn’t alone. In chapter 445, Kaioshin explains that there used to be five Kaioshin, but the others all fell to Buu and so now he’s the only one left. Later in chapter 508, he further explains that these five Kaioshin were divided up in the same way as the Kaio: a Kaioshin of the North, South, East, West, and then a Grand Kaioshin above them all. The lone surviving Kaioshin was actually the East Kaioshin back then, and in chapter 479 we’re introduced to his ancestor, the old perverted East Kaioshin of 15 generations ago.
So for each Kaio, there’s an equivalent Kaioshin (or there used to be, anyway). In the main series it’s never really explained what the point of this somewhat odd setup is, but later guidebooks like the Daizenshuu say that while the Kaio just watch over the living world, the Kaioshin watch over the afterlife as well. When Toriyama drew the map of the Dragon Ball cosmos for the Daizenshuu, he took the design he had come up with earlier for the afterlife tournament filler arc, but expanded it to include the Kaioshin’s realm as well (a process he explains in his Daizenshuu 4 interview). While Dragon Ball’s living world and afterlife are crammed inside a single huge ball, the Kaioshin’s realm gets a separate ball all its own, which revolves around the other one like a moon so that they can monitor it. A nifty idea, though one that seems like it arose mostly from the way Toriyama tacked the Kaioshin’s realm onto his pre-existing cosmos design.
The apparent redundancy highlights the difficulty of fitting a pantheon into Dragon Ball’s world of shonen-style constant power inflation. Once that initial Dragon Ball hunt wraps up, the series is all about getting stronger: the characters are always going through more intense training, reaching more powerful forms, fighting tougher and tougher enemies, etc. For the most part, Dragon Ball’s gods are there to provide Goku and co. with training, quickie power-ups, and the occasional heads-up about the next imminent disaster. Then when an even bigger threat turns up, Toriyama introduces an even higher god to provide the characters with even better training and/or power-ups. In this light, Dragon Ball’s gods are all just bumped-up versions of the more down-to-earth mentor figures like Kame-sen’nin or Karin, and even these guys are mystical immortals.
Theoretically we get introduced to new gods whenever Dragon Ball’s worldview expands (more planets=more gods; more galaxies=more Kaio, etc)…but in practice, new gods turn up whenever the series’ power inflation has rendered the old gods useless. The end result is that in the Buu arc we get the Kaioshin because at this point nobody would buy it if regular old Kaio suddenly owned magic fusion earrings or could do a funky chicken dance that made Super Saiyan forms obsolete. But there’s no expansion of the Dragon Ball world to go along with this like we had in earlier arcs, so we get new gods without anything new for them to be gods of, apart from their own realm which exists solely for them. They’re around mostly for their plot function, with only a hazy in-universe reason to exist.
It seems Toriyama was aware of the issue, since when creating a new addition to the DB pantheon for Battle of Gods, he did as much as possible to separate Beerus from all the previous Dragon Ball gods. Even before Toriyama got heavily involved in the project, the initial idea for the first DBZ movie in 17 years was that it would feature a “God of Destruction” and a “Super Saiyan God.” Once Toriyama started rewriting things, he quickly put his own distinctive spin on these ideas. Rather than a pure evil villain as originally planned, Beerus ended up a more neutral figure, selfish and ill-tempered but not actually malicious, with many humorous aspects to his personality. As already mentioned, Toriyama designed Beerus as a cat man to separate him from the other more human-looking gods, and his feline nature shines though in his laziness and obsession with food.
One phrase that got kicked around a lot in the previews and other early press for the movie was that this God of Destruction was needed “to maintain the balance of the universe.” As Toriyama explains in his Chouzenshuu 1 interview (released a few months before Battle of Gods itself), Kaioshin is actually a “God of Creation” who “provides the stimulus” for new planets to be born, and even creates new life-forms...though he mostly just watches over all these worlds, with help from the Kaio (this is the only aspect of their role touched on in the original series itself, or earlier guides). Since the number of planets is constantly increasing, Beerus provides balance by destroying worlds. It’s a bit like hunters shooting deer to keep their population in check. Of course, Toriyama never explains why Kaioshin couldn’t just stop making planets if there are getting to be too many, or why too many planets would even be a problem (can outer space get overcrowded?), but the important thing is that Beerus has a clearly-defined role. In the movie itself, Kaio gives Goku a shortened version of this expl
anation, saying that both creation and destruction are needed to preserve balance (though he doesn’t touch on the whole “planetary overpopulation” idea).
But the biggest difference between Beerus and prior gods is that he’s an antagonist; not strictly evil of course, but someone there for Goku to fight all the same, as opposed to serving as a mentor or martial arts master. Wrapped up in this is the idea that to fight Beerus, Goku must himself become a god, which is where Super Saiyan God comes into the picture. Like with the God of Destruction, Toriyama put his own spin on this. Rather than a super muscular guy in a cape, as was originally planned, Toriyama wanted to keep things as simple as possible: Goku is skinnier and younger-looking, with red hair and a fiery aura. The backstory (as explained in the movie by Shenlong) describes this form as the “god of the Saiyans,” originally created long ago by the few good-hearted Saiyans to oppose the wanton violence of their peers. Five good-hearted Saiyans have to share their energy with a sixth to trigger the transformation.