Daémon are based on the daemons from His Dark Materials, except that these work under different underlying mechanics and take the forms and abilities of Pokémon instead of earth animals. They make up the "magic system" of the setting Daémon.
They are the souls of their human. They appear from nothingness upon birth and vanish upon death. Killing one of a pair will kill the other. If one of a pair is in pain the other can feel it, and emotions are shared.
Daemon are sapient and capable of speech regardless of the shape of their mouths (or even lack of mouth, in the case of staryu and similar). They tend to have more fanciful names than their human.
While alive, they appear to be made of physical material, have the expected properties such as weight and mass, and interact with things and phenomena as expected of matter. They do not need to breathe, though do feel uncomfortable if unable to. They do not need to eat and do not feel hunger. They sleep at the same time their human does - having one awake and one asleep is very difficult and will typically correct itself one way or another after a minute or two. Upon their death, if they have swallowed something or had an implant of some kind installed it will be left behind rather than vanishing with the rest of them.
Daemon can take the form of any pokemon, including variants such as Alolan vulpix. They do not mega evolve or dynamax except as a possible interaction with non-local magic systems.
Legendaries are much rarer than any other type. About 1 in 100,000 people will settle as a Legendary, or about one in 7 million per legendary species. Other pokemon will settle at roughly similar rates to one another. Some like rattata are a bit more common, and others like the Ultra Beasts are rarer, but the rates are all much closer together than between normal forms and legendaries. Shiny pokemon are relatively uncommon, appearing one in 4096. There is no genetic trend for settling in a similar form to one's relatives.
Getting a Daemon
People are naturally born with daemons if their mother had one. These appear in the form of Ditto nearby upon the baby's first breath.
The relevant point of soul variant inheritance is during pregnancy, so it is the womb-mother not the gamete-mother when these are different. Species which hatch from eggs or attempts to use artificial wombs can not transfer this variety of soul - they will have whatever soul is natural to the universe they come from. Otherwise, daemons will overwrite
There had been people with different kinds of souls on the Dead World, long ago, following the same inheritance pattern. These other soul varieties went extinct thousands of years ago, and by the time the Fleet launched there were no reliable records beyond what can be reasonably assumed to be fiction.
It is possible to attach a daemon to a human in a variety of ways. The only known method available with the Fleet's technology is in the hands of an illegal lab which also has access to an interdimensional portal which they can steal test subjects with. It involves having an existing pokemon and binding it to a person.
Daemons can also be removed. The only technology known to the Fleet to do so is Intercision, which turns the daemon into a non-sapient pokemon and causes the human to permanently lose their soul and suffer effects similar to being lobotomized. This is a suppressed technology, also only used by the illegal lab studying daemon attachment.
All daemons start as Ditto, only able to transform temporarily into forms they see at the time. During puberty, a person will Settle into a permanent form. They don't need to be in the form or have anyone nearby with that form to settle. This happens between the ages of 10 and 18, most often between 11 and 13. Some people stay ditto, at about the same rate as any other non-legendary. Settling is not a choice - they can not choose to settle or choose to refuse, or pick what they settle as.
All ditto are indeterminate gender. If a person's most suitable form is one which does not have a gender or only has one (Tauros, Jynx, Magnemite), their daemon will have that gender. For everyone else (rattata, eevee), they are significantly likely to settle to match the gender of the humans their human is most attracted to.
Daemons always feel that their settled form is physically correct for them. They may have issues with its cultural connotations or practical considerations.
Daemon do not age, but they can suddenly shift form in a phenomena called Evolution. (No relation to the Darwin thing)
If someone strongly fits an evolved pokemon but not the previous forms, they will evolve quickly over the course of only a few years. A rare few will settle as a second-stage evolution from the beginning. Many people reach a stage that feels correct then never evolve past it - there are plenty of people in their 70's with first-stage daemons. Others evolve later if their personality shifts in the correct way.
If a human and their daemon get more than a few meters away from each other, they experience pain and distress. The pain gets worse the further they go, or the longer they remain apart. This includes a feeling of doing a great harm to oneself. The point where the tether snaps is about 1 kilometer.
The proper ritual of separation is to lock the daemon in a cage they can not escape from, then the human walks away. Only 2% of people attempt this, and half fail and never retry. It is mostly common among people working as emergency responders or have unusually large daemons. A daemon will never willingly walk too far away from their human, and will always attempt to reach them after a point no matter what their personality is.
Separation via walking is not dangerous. Separation by being dragged away from a daemon can be dangerous, and becomes more dangerous the faster it is done. Even teleporting out of range (the most risky method) is not guaranteed fatal. A daemon who has been separated may be angry with their human for a while. There are occasional short term emotional issues reported.
Even those who can travel away from themselves rarely will - there is a feeling of being in two wildly different circumstances which is distracting and unpleasant. While on the Fleet, there is also the cultural issue of not being able to have both halves of a conversation and the expected specializing between halves.
(Note that this is different from Intercision, which tears out the soul.)
Daemons may touch other daemons without magical effects. Humans may touch other humans without magical effects. A daemon can touch the human that they are the soul of without magical effects.
A daemon who touches a human that is not their own will experience a strong sensory reaction that is most likely interpreted as pain. It is a feeling like that person is Everywhere.
Theoretically, two people who are very close and truly trust one another completely will not mind this overwhelming closeness. This is almost never done, even among people who are married.
Short accidental touches do happen in crowds, and are still unpleasant but Accidents Happen. Steps are taken to avoid this. Intentional non-consensual non-self contact is a crime and punished very harshly.
Certain psychic pokemon will automatically read the minds of humans who they touch. They still have the unpleasant EVERYWHERE effect of any non-self contact, but it includes additional information. This typically makes it even less pleasant.
[list of psychic pokemon with this effect: ralts line, hattena line, [incomplete]]
Barrier and Fainting
more emotive, less logical than their human
Instinctive desire to battle.