Familiars are beings summoned by Nobles (Mages) of Halkeginia, and are seen as one with their master/mistress. Through a ritual the creature most suited for the mage in question will be pulled from where they are and bound to the mage for life. In addition, the familiar will be slowly molded by magic into the best possible person/being for their summoner; as shown when Saito got comparatively more courageous and bold while slowly losing his will to want to return to his home world.
According to Derfflinger, he says that it is impossible to re-summon the same familiar again. However Louise breaks this rule and Saito comes through the summoning portal twice.
Louise's summoning line:
- Anime version: "My servant that existed somewhere in this vast universe, my divine, beautiful, wise, powerful servant, heed my call, I wish from very bottom of my heart and add to my guidance and appear!"
- Manga version: "My name is Louise Françoise le Blanc de la Vallière. Pentagon of the five elemental power, Heed my summoning... and bring forth...my familiar!!" Manga contract "Pentagon of the Five Elemental Powers, grant your blessings upon this creature and bind it as my familiar." Then she kisses him.
- Novel version: "I, Louise Françoise Le Blanc de La Vallière, in the name of the great Five Pentagon Powers, following my fate, summon a familiar." Contract: "My name is Louise Françoise Le Blanc de La Vallière. Pentagon of the five elemental powers; bless this humble being, and make him my familiar." at which point she kiss him to seal the contract and the runes appeard.
Familiars have a compulsion to serve the mage to their best extent, and sees their master/mistress as someone they must serve. Even for humans this is no exception, although the effects are varied. Julio serves Vittorio Serevare without question, but the same cannot be said for Saito and Sheffild at times. Even for rhyme dragons, ancient dragons who can also use 'Magic of the Firstborn', are no exception as Sylphid obeys Tabitha.
There are no rules that a familiar cannot kill his/her/its summoner, as shown 6,000 years ago when Brimir's familiar, Sasha, stabbed him through the heart with Derflinger to prevent the extinction of the elven race. Another example is when Sheffield stabbed Joseph and then turned both of them into ash using an artificially made Firestone that made a ball of fire many leagues in radius, after she realised Joseph did not love her the way Louise truly loved Saito. Though the compulsion to serve the summoner makes it harder, if not impossible, for a familiar to kill its summoner. Void Familiars are unlike other familiars, in that only humans or humaniods can be summoned. In addition, regardless whether the Void Mage casting the spell is not the master he/she is bound to, the Void Familiar will feel the same way as if it is his/her own master chanting. When their master is chanting, the familiar feels more courageous and have more power running through them via the familiar runes.
6,000 years before the current events of the Light Novels, the summoning ritual does not exist, and runes has to be carved onto the familiar itself. Founder Brimir is the first to have a familiar, carved onto an elf named Sasha, when he was experimenting with magic. His village at the time was desparate in a fight against a superior species who had metal armour when humans at the time only lived like Mongolian nomads, and due to needing strength he carved those runes on Sasha but forgot to mention the summoner/familiar binds that come with it. Later on Brimir created the familiar summoning ritual in his lifetime.
In the case of Void Familiars, there are two possible reasons why a person becomes a familiar. One is Destiny, and the other is Love. This was discovered when Brimir summoned Sasha as the 'Heart of God', despite she is already his Gandálfr, since at the time Brimir had fallen in love with the elf. Familiars ending up being 'resummoned' this way happens twice more in the series: Saito being resummoned into being Tiffania's familiar and holding the addition power of the 'Heart of God'; and Julio being resummoned as Tabitha's twin sister's familiar, taking on the powers of the 'Mind of God' in addition to his Windalfr power.
When being resummoned, the previous obligations and powers acquired when bound to the first summoner does not disappear. Also, when it comes to 'Love', it is only necessary for the summoner to have that intense emotion. The familiar being in love or not doesn't seem to be a factor.