The mythos of legendary pokemon in Kasei is a complex one. With the sheer diversity of legendaries that can be encountered within the region, most creation myths have been lost to history. Still, in recent months, breakthroughs have been made in the study of legendary pokemon, resulting in a largely-accepted classification system that separate the legends of Kasei into two groups.
Our first group, the Primes are the 'gods' of legend of which stories are written. These are the True Legendaries, who use their immense power to govern their elemental domains, maintain the time/space dimensions, provide the sweetest of dreams or the worst of nightmares, grant grandiose of wishes, or even move continents. The nature of the Primes remains a mystery due to the fact that no documentation of an actual encounter exists, but it is believed that the Primes choose to stay out of human affairs. If you believe that you've come across a Prime somewhere on the road, you probably haven't. That, of course, isn't to say that trainers don't come across legendary pokemon in the their travels...
The second group, Betas, are the legendaries that trainers are most likely to come across during their journey. There are two very different theories as to why Betas exist in the first place, but one thing is certain: Betas only possess a mere fraction of their parent legendary's ability. One theory regarding Betas is that they are simply extensions of their Prime's power, acting as proxies to assist their Prime's role, but in a diminished capacity. Another, more widely-accepted theory is that Betas are simply a defensive mechanism, employed by Prime legends to distract would-be hunters from disturbing them, and potentially causing a global catastrophe by upsetting the natural balance of the world itself. The number of legendary-specific Betas in existence at any time is largely unknown, but researchers believe that it varies based on the whim of the Prime in question. This is the exact reason why a single trainer can be possession of multiple specimens from the same species.
Betas and their Primes: Despite being separate physical entities, upon their 'birth,' Beta legendaries in the wild are created with a subconscious directive, leaving them with the faint urge to mimic their respective Prime's duty. (IE: A Beta Dialga being drawn towards minor temporal disturbances, or Tornadus' inclination to cause weather-related mayhem.) However, when a Beta meets and bonds with a trainer, or a fusion occurs, that programming is quickly overwritten by its own will, which explains why legendary-fusions aren't overly burdened by their Prime's compulsion.
Duos, Trios, and their Masters.: There are many legendaries who are part of a larger group, be it the Eon Duo, the Creation Trio, the Lake Trio, or the Lunar Duo, to name a few. Betas are no different. It is unknown how the Primes manage to coordinate the act, but no one Beta of a legendary group is born alone as an entire group is created together, in the same instant. Members of a group share an intrinsic bond that never seems to dissipate. Additionally, unlike the Prime's compulsion, this connection persists even through fusion or trainer's bond. For example: for every Beta Latias, there is a Latios whom she regards as her brother, and the same applies to the other duos, such as the Lunar pair of Cresselia and its sibling-Beta Darkrai. In the case of Beta trios, the group is governed by a Beta of that specific trio's master. Because of this, it is theorized that there are exactly the same number of each groups' members in existence at any give time around Kasei.