A familiar (使い魔 tsukaima) in the Nanoha universe is a magical creature that is created by a mage with an animal.
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 Rynith 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 his/her familiar begin to share most of their powers, such as knowledge of magical spells.
In Alph's case, Fate Testarossa created her when Alph's former animal body was on the brink of death from infection. Since it was an act of selfless kindness, Fate did not have a specific purpose for Alph in mind and later granted her full freedom as her part of their contract. Alph, however, then voluntarily swore to serve as Fate's protector for the rest of her life. On the other hand, Rynith, at least in the movie continuity, was a healthy animal who was directly "transformed" into a familiar by Presea Testarossa.
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. Alph is voraciously carnivorous) and possess the ability to shapeshift into their animal form. In their humanoid form, they will normally have the ears and tail of the animals they are based on but, if necessary, they 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 Alph and Zafira learn to shapeshift into puppies, and by the time of StrikerS (when she has retired from combat) Alph 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 A's (ep.5), Zafira insists that he is a "guardian beast" (although Alph retorts that there is no difference), but later he refers to his rival as a "fellow guardian beast". Alternatively, the difference may simply be that of nomenclature, just as armed Belkan system practitioners are called "knights" as opposed to Midchildan "mages", despite the fact that both are users of magic.
|Alph||Fate Testarossa||Midchildan dog/wolf|
|Aria Liese||Gil Graham||Cat|
|Lotte Liese||Gil Graham||Cat|
|Rynith||Presea Testarossa||Mountain cat|
Although Yuuno Scrya is thought to be Nanoha's familiar at times, he is a mage with a talent in shapeshifting despite his frequent appearances as a ferret. 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.