Void

Void Magic is a lost magical element branch which is very powerful. It is based on the fifth of the Japanese elements namely "Kyu" (Void) but it is renamed as "Kyomu" in the anime.

Void magic, as implied in the series, is an element which does not focus on elemental magic (i.e. healing, alchemy, etc.); rather, it mostly gives off destructive spells and magic, like Dispel and Louise's Explosion spell.

Void, however, despite its terrible impact, it is actually a useful element as it is the only element to match and be able to clash against Earth's weaponry.

Since it is very rare, there have been only four Void users and familiars alive for a long millenia. These (theorized by this user) users and familiars are:
 * King Joseph de Gallia, the current king of Gallia; His familiar is Sheffield, the Myozthirirn, or the Mind of God.
 * Louise Francoise de La Blanc de Valliere, also known as Louise the Zero, a Noble and second-in-line for Tristain's throne (Henrietta's proclaimed sister); Her familiar and most loyal protector (not to mention her most important person) is Saito de Chevalier Hiraga, the Gandalfr, or the Left Hand of God.
 * The Unnamed Pope of Romalia; In the light novels, it has been implied that Julio Caesar is his familiar, the Windalfr, or the Right Hand of God. However, none of this was explored in the anime: Julio was shown without a master, the Windalfr was never mentioned, and the Pope never actually appeared.
 * The last one is probably Tiffania because she has memory wiping magic, revival magic, and probably ancient elven magic which could be related to Void magic. Also, she has knowledge of the Void magic, its users and their familiars. She had once accessed Jospeh's music box, which she plays as a song in Princess no Rondo. As she is a half-elf and also Albion royalty (Henrietta's first cousin; Tiffania's father, the Archduke of Albion, is Henrietta's father's younger brother), it is probable that she does not have a familiar. She might be the Heart and Flute of God (she mentions the Heart and Flute while singing the song Joseph's music box also played, which proves her Void knowledge)

Trivia
 * It is interesting to note that Tiffania might actually be the Heart of God because she has a tender heart, not hesitating for another's sake when in danger or using her own magic to help the gang.
 * Sheffield's magical creatures seem to have a same rune that glows like hers.