Saito Hiraga's activated Gandálfr runes
Gandálfr, also known as the Left Hand of God, is a legendary type of familiar, with only two known familiars of this type, including one of the legendary Founder Brimir's own familiars and Hiraga Saito. It is notable for being the sole type of familiar of Void mages. A Gandálfr's primary combat duty is to guard the Void mage as he or she slowly casts Void magic. Gandálfr are almost as legendary as their Void-magic using masters, and are even known to be able to take on armies of thousands alone with their abilities.
They are unique for being completely human, unlike other common magical or beastly familiars. The only indicator of their being familiars are the rune markings located on a part of their body.
The primary power of a Gandálfr is absolute mastery of any item meant for combat, from swords and pistols to artillery cannons and fighter aircraft (though in the light novels, even Kirche's ceremonial sword gift to Saito activated this power). On physical contact with such weapons, the Gandálfr's runes will glow, and the familiar gains heightened strength, agility, endurance, and reflexes in addition to complete proficiency over the weapon. These reinforcements linger only for a while when the power is deactivated; a Gandálfr who sustains normally fatal damage while powered up will quickly face the brunt of the damages when he returns to normal, which may cause him to collapse out of exhaustion or even possible death.
According to Julio, Gandalfr's will have signatures, which is any weapon deemed the most at the era. For example Sasha who wields sword and spear, and who Saito wields Sword and Gun
Although it is not stated, it is hinted that Gandalfr also has mastery over scientific application of technology to be included in using any weapon. This side-effect ultimately forges the familiars body into that of a warrior, as seen with Saito despite being a brain-type human.
Also, a Gandálfr has sensory connections with its master, allowing both master and familiar to see or hear what the other does, knowingly or unknowingly.