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. That 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 prone to come across in their travels. 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 roll, 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.
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 maintain their Primes. (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 burdened by its 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, the Lunar Duo, and 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; an entire group is created together, in the same instant. Members of a group share an intrinsic bond that never seems to dissipate.