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Closely linked to the spell used on a Bound Summon, the Familiar Uplift has survived to the modern day almost intact. In Alhazredian times, it was limited to the nobility, who used it for their personal servants made so that others could not compromise them. It is in effect a brute-force Uplift, applying a template of knowledge and intelligence taken from the mind of the caster to a non-sapient creature (which must be a 1-life mammal, though a variant exists for 1-life birds and bird-like dinosaurs *), using the brain of the animal as a structural shortcut so the incredible complexity of creating a new intelligent being can be done by relative primitives, rather than the high end tech needed for an ID.
This is sometimes done with an animal that has its own Linker Core, but not often - such creatures are uncommon, and their existing Linker Core interferes somewhat with the Uplift, making it more difficult to cast. The familiar cannot know anything that the caster does not, but they will come into existence at a maturity level approaching that of their master, and with all the knowledge they have been bestowed with ready for use. Loyal servants and valuable support mages, they are incredibly useful to those with the magical capacity to required support them - which is intimidating. Gil Graham, to support two Familiars of the level of Aria and Lotte Liese, was a monster. Arf is the obvious example in-series, and it is because of the nature of the spell that she is no more mature than Fate is, and indeed somewhat less so. Finally, Linith shows what a Familiar created by a powerful, knowledgeable and skilled mage looks like - and indeed, Precia was something of an expert in familiar research, so Linith is a glowing example of her kind.
Familiar law has the familiar as a dependent with slightly less rights than a child. The master is obliged to the familiar as a dependent, but on the other hand, to be frank, they've never managed to ban sexual relations between masters and familiars because of the loyalty stuff. It's ethically controversial, but the familiar will always say that it's fully consensual, and they can be adult in mentality.
It is outright illegal under TSAB law to destroy a familiar once they're made; they're for life, not for Christmas, as a fully sapient being. It's also impossible to "unhook" them from someone, else Fate might well have done it in StrikerS to remove Arf's dependence on her (Arf would certainly have at least asked). However, you need large reserves to support a True Familiar - Nanoha and Fate are only a little way above the cap at which point it becomes impractical, really - so it's less common than you might think.