Selene describes herself and familiars in general as:
“An entity created by the alien race you know as the “Zenith.” As for what I am, the closest equivalent humans have is that of a highly advanced artificial intelligence. Although the proper explanation is much more complicated, you may think of me as an AI formed by both magic and unrivaled technology. I was created in order to assist humanity in their fight against the Anathema. Humans have taken to calling us “Familiars.””
Later stating that “Each Familiar is personalized from inception to be a match for their potential Guardian”
If a candidate declines to become an MG, the familiar becomes an ‘unbound’ familiar, and does various tasks to assist guardians. For example, “unbound Familiars with the Association will have begun taking measures to help maintain your [Mai's] civilian identity.” Selene, ch 36
“Familiars act as a Magical Guardian’s personal assistant to not only guide them through their new abilities but also aid them in whatever they might request. Essentially, you will have a magical AI in your head whose only purpose is to help you for the rest of your life. That is why Familiars are personally customized to each Magical Guardian candidate. Extensive information gathering was done to make sure our personalities would be complementary and so I would be aware of the various aspects of your life.” Selene ch.3
Familiars are “magical constructs". They have levels. Their level is locked to their MG, although normally it does not mean much; unless their MG spends points on them, Familiars do not grow stronger with level. (Mikasune, discord, #mgg-general 4-1-2024. And no, this was not an April Fools joke.)
Summoning the physical form of a familiar costs 150 Mana at level 1, this cost increases by 10 mana per level. A minimum cost summon will generally last for one hour, though the duration can be extended by spending more mana. Also through the use of excess mana regenerated when their MG’s mana is full, which allows the familiar to stay in physical for indefinitely.
Of note is that familiars do at least a part of the research on their potential future guardians by following them around while invisible for a period of time. And it seems like they have input if not the final say on if someone becomes an MG.
All familiars have the following five laws imposed upon them (ch. 37):
“One: Familiars are subservient to their Guardian. They are to guide with faithful advice, not make their decisions for them.”
“Two: Familiars can conceal information from their Guardian if they believe it is in their best interests, but they may never outright lie to them unless it is to follow another restriction.”
“Three: Familiars may not spy on other Guardians for the purpose of finding out information about them, and they may not share other Guardian’s personal information, especially anything that might be used to uncover a Guardian’s civilian identity. ”
“Four: A Familiar may not attack and dissipate the summoned form of another Familiar. They also aren’t allowed to point out other Guardian’s Familiars when they are using stealth magic to hide themselves unless they are interfering with their Guardian in some shape or form.”
“Five: Familiars must obey the orders of their Guardian and may not work to undermine them or go against their intentions even if they are unstated. The only exception is if following their Guardian’s orders would violate one of their other restrictions.”
Familiars are unable to read the mind of their MG, and can only hear the MG’s thoughts when specifically directed at them.
Familiars share the access level of their MG, and are capable of interfacing with most magitech items.
Familiars have access to the dossiers of all active MG that contain information on their public activities, as well as anything else they have made available.
Familiars will normally be housed in their MG’s soul gem, but can also be summoned into a physical form.
Said form can move very quickly, become selectively invisible to most methods of detection, as well as communicate telepathically with their MG (exact range unknown).
It is also common for MG to summon their familiar to allow them to roam and gather information while out of astral shift. When in physical form they are unable to do any damage unless certain upgrades have been purchase, as they are essentially a stuffed animal. While in physical for they cannot feel pain but can be temporarily killed that said familiars are essentially immortal.
The only way to kill one is to destroy the soul gem in their MG’s skull. If the MG they are contracted to dies, and the soul gem is still intact. The familiar will be summoned on the spot with the soul gem inside them and have until they run out of energy to find a new source of energy. Only if they can’t find one will they die.
Familiars are also able to phase through most physical objects so long as they are stationary and not too thick. Doing so destabilize their physical form and reduces the summon duration.
If a familiars physical form is destroyed there is a 2x cost mana penalty to resummon them that lasts for 1 hour.
Familiars have an alternate, very important function: to serve as a squeeze toy for their MG. (ch. 36)