There is only ever one True Successor for a Legendary.
That is to say, only one person is able to survive the transition into Successor to best serve their Patron, and this person only comes around when a time of need arises. Dire and desperate need, or when the Legendary feels their time may soon yet be at an end…or whenever they damn well feel like seeking out their True Successor, honestly. Deities are weird like that, they’re fickle creatures. Some Successors have wandered the Earth since the dawn of time itself alongside their Patrons, and some yet are still just being born and brought into being to serve their deity. Of course, each universe only has a set few Successors–there are only so many Legendaries, after all. Each gets one, and only one, True Successor. They do not reincarnate, they are not reborn, they come once, and only once. Timeless deities they may be, but they are not invincible.
Some Legendary Pokemon end up cycling through many, many humans before they actually find their True Successor, the one born to serve them and succeed them. This person is someone that shares a kindred spirit with their Legendary. That is to say, a person who is born with the same innate beliefs, personality, and ways of thinking as their Patron. Someone who is born on the same wave length as their Patron, someone who will be able to understand the Legendary on a very base level, and the Legendary them. Cut from the same cloth, molded from the same spirit, one in the same as each other.
True Successors cannot be made or trained into True Successors. They come only when they must, inevitably drawn to the one they are meant to serve under. That isn’t to say that a Successor must follow their Patron religiously, goodness no, the Alpha Successor themself does not adhere to their Father spiritually. But it does mean that the Successor will follow their Patron without question upon meeting, offering their aid and support as need be–often times to the confusion of the Successor. A Successor’s duty is to help the one they follow in their own tasks. Yveltal’s Successor would be there to help souls along so none linger, Mew’s Successor would breathe new life into the world, so on and so forth–but only at their Patron’s side.
There are humans who are inherently capable of serving a Legendary and aiding in their tasks. But not to the same extent that a Successor is able to carry these duties out. A Successor is one who naturally flows with what is expected of their Patron before they are granted any abilities. Working in sync with their Legendary to lessen the load their Patron carries, accepting that burden unto themselves without question. Only the tiniest fragment at first, of course, a human’s body is not capable of handling all that a Legendary’s does.
A Successor flows perfectly with their Patron.
They have to, else they aren’t their True Successor.
The Successor serves one purpose and one purpose only. To ensure balance is kept should their Patron die. A True Successor is able to survive the transition into a variation of their Patron, holding a [???] Typing to signify their Successor status. A watered down version of their Patron, meant to be trained and grow stronger over time to be an effective contingency plan. To hold things in place until the next True Child can be created by Arceus ( or even the Alpha Successor ) to take over again–which can take hundreds, even thousands, of years to accomplish. This process is a slow one, a careful one, to ensure that balance is never once disrupted. It can prove potentially catastrophic with those that hold the most weight upon their shoulders, after all. Such as the Creation Trio.
The transition in and of itself, however, cannot be survived unless the Successor is extremely close to their Patron. They must be the closest of friends, perhaps even like a parent and child, or siblings, maybe soul mates, for the Successor to be able to live through their first transition in to their True Nature. A tiny, newborn Legendary. It is for this reason that most True Successors have the common sense to wait many, many years before attempting this shift. The bond the Successor and Succeeded must share has to be unbreakable, unshakable.
Once this change is made, it may take some time for a Successor to be able to maintain a human guise again. It eats away at one’s energy reserves to hold a human form after the change has been made, and a Successor is notably weaker in their previous form than they are their True Nature. Albeit, their strength is proportionate to the strength of their Patron. One should not mindlessly pick a fight with the Alpha Successor, for example, no matter their form–they are still yet capable of raining down Arceus’ own fury down upon their opponents even without being in their True Nature.
Overall, however, a Successor serves only their Patron and both the Alpha Pokemon and Alpha Successor. Depending on their Patron’s status, this may also include other higher ups in the Chain of Command. Mythic Pokemon and Mythic Successors, for example, fall well below the tier of major Legendaries and their Successors, and are expected to show respect and support towards those who take on far more duties in maintaining the world’s balance than they.
It’s also considered extremely rude among Legendaries to show disrespect towards one anothers’ Successors. A Successor is one who turns their back on their humanity, their life, all that they know and love, solely to serve as a martyr should they ever be needed. They are willing sacrifices for the sake of maintaining balance in the world, all Patrons tend to be fiercely protective over not only their own Successors, but the Successors belonging to others as well. These are individuals who give everything up for the sake of maintaining balance and adhering to their respective Patrons without question.
Successors are the children, the unyielding servants, of the Divine.
Considered the most loyal followers of their Patrons, even if it not religiously.
The Alpha Successor watches over them well, as a Matron.
Training them, guiding them, leading them. Preparing them all for their own transitions into True Successors, for being able to handle their True Nature, in which case their training then falls to their Patrons. For their Patrons to ready them should the worst ever truly come to pass, to make their bodies strong enough to handle the cosmic weight that comes with a Legendary’s full duties in the world should the one they serve die.
Demigods who are demigods solely as emergency backup in the face of crisis.
That is the fate of a Successor.