A familiar (使い魔 tsukaima) in the Nanoha universe is a magical creature that is created by a mage, based on an animal form.
Creation of a familiar involves extracting an animal's soul and infusing it into a magical body sustained by the mage's mana. The familiar then, as Linith puts it, "shares one heart" with its creator, meaning that their lives are henceforth linked. Creating a familiar does not automatically guarantee its loyalty, though, so a familiar should normally be bound by a contract with its master. This agreement grants it a specific purpose and it will vanish or expire once this purpose is fulfilled. On the reverse side, after "signing" the contract the mage and their familiar begin to share most of their powers, such as knowledge of magical spells.
In Arf's case, Fate Testarossa created her when Arf's former animal body was on the brink of death from infection. Since it was an act of selfless kindness, Fate didn't have a specific purpose for Arf in mind and later granted her full freedom as her part of their contract. Arf, however, then voluntarily swore to serve as Fate's protector for the rest of her life. On the other hand, Linith, at least in the Movie continuity, was a healthy animal who was directly transformed into a familiar by Presea.
In general, familiars are fiercely loyal to their masters (to the point of breaking the law) but may have the liberty to question their motives. They apparently retain some instincts of their past animal lives (e.g. Arf is voraciously carnivorous) and possess the ability to shapeshift into their animal form. In their humanoid form, a familiar will normally have the ears and tail of the animal they are based on but if necessary, are capable of hiding these features.
Familiars are empathically linked to their masters and dependent on their magical power. The more lifelike and complex the familiar is, the greater the necessary power upkeep. It follows, then, that a familiar will perish should their master die. A familiar can, however, significantly reduce mana expenditure required for their upkeep by reducing their body size, e.g. both Arf and Zafira learned to shapeshift into wolf puppies, and by the time of StrikerS (when she has retired from combat) Arf has switched her humanoid form from an adult to a child. The light novels reveal that the master may choose to sever the empathic link.
The use of familiars may be an exclusive feature of the Midchildan magic system. In episode 05 of A's, Zafira insists that he is a "guardian beast" (although Arf retorts that there's no difference), but later he refers to his rival as a "fellow beast of protection". Alternatively, the difference may simply be that of nomenclature, just as Belka system practitioners are called "knights" as opposed to Mid-Childa "mages", despite the fact that both are users of magic.
|Alph||Fate Testarossa||Midchildan wolf|
|Aria Liese||Gil Graham||Cat|
|Lotte Liese||Gil Graham||Cat|
|Rynith||Presea Testarossa||Mountain cat|
Although he is thought to be Nanoha's familiar at times, Yuuno Scrya, despite his frequent appearances as a ferret, is a mage with a talent in transformation. Likewise, while Friedrich may appear to be Caro Ru Lushe's familiar, he is actually a summoned adult dragon, whose true form is permanently sealed under normal circumstances.