A world situated upstream of Midgard (also known as Earth). As a rule there is no route linking the two worlds, but through the rarely-occurring “Bound Collapse,” humans of Midgard sometimes “fall” into Elcrest. The inhabitants of Elcrest are said to have been formed by people who fell in from other worlds in this way (Stellas can travel between the two worlds by using Formula Arts).
 Elcrest is forever menaced by a threat called the “Shouki,” which wells up from “Ninom,” the world set back-to-back with it. The maximum strength of the Shouki that appear is proportional to the years that have passed. For that reason, those who stand against them have always had to grow ever stronger as well.
 Elcrest’s society has a wholly two-tiered structure. The “Civilians,” who have no combat ability, build colonies and live clustered together, and the “Stellas” protect those colonies. In return for protecting the Civilians, the Stellas receive various benefits from them.
 Their cultural level is not much different from Midgard’s, but in science they lag somewhat, since they follow in Midgard’s wake. On the other hand, they have advanced “Formula Arts,” a technology that uses an energy called “ether” as its medium, and it is this that makes opposing the Shouki possible.

 Those who protect the people of Elcrest. That sounds noble enough, but in truth they are beings fated to spend their every waking day in battle.
 Each Stella belongs to a power bloc called a “Hamlet.” A Hamlet holds the colonies where Civilians live as its territory, and in exchange for protecting them from the Shouki it receives sustenance and various services. This resembles the relationship between a ruler and the ruled, and it is not rare for Hamlets to come to blows over colonies. Yet ultimately a Hamlet and a colony are equals, and what defines the relationship between them is a “contract.”
 Stellas are required to be strong at all times. This is shown plainly by the fact that no common law exists to protect a Stella’s body or mind. As a rule it is the Stella herself who must protect herself, and even if she is captured by an enemy and subjected to any torment whatsoever, there is no law to rescue her, nor any to punish her captor (she does, however, retain the freedom to take her own life, and this stands as the sole deterrent against those who would seize her). Most Hamlets forbid inhumane acts against the Stellas in their ranks, but there also exist Hamlets that hold pleasure and destruction above all else (such as the Shattered Chalice).
 In ordinary times each Stella acts by her own order of priorities, but in the face of a crisis that threatens the world they are commanded to unite as one and stand against the threat. The ones who bring this about are the neutral bodies—the “Senate,” the “Council,” and the “Joint Command”—together with the “Oversight Board,” whose role is to watch over them.
 The ratio of men to women among Stellas is roughly 5:95, owing to the tendency for women to possess higher ether potential (the basis for this is presumed to lie in the sex chromosomes, though it has not yet been clarified).

 Beings that well up from “Ninom,” the world set back-to-back with Elcrest, and pose a threat to its people. In Ninom all entropy decreases, and these low-entropy existences leak out through a distortion in the world—the “Bound Shift”—to manifest within Elcrest. The expanse of Ninom is theoretically held to be infinite, and the precipitation of Shouki via the Bound Shift is thought to follow quantum-mechanical probability. In other words, as time passes the seeds of Shouki present in Ninom grow ever more formidable (though, by the theory just mentioned, the maximum strength of the Shouki is said to rise only linearly).
 Should a Shouki precipitate so powerful that even the Stellas banded together could not defeat it, Elcrest would in an instant be turned into a world of death. This is the foremost reason that strength is demanded of the Stellas.
 However, being low-entropy bodies, the Shouki are also interchangeable with a source of energy. The Stellas use conversion Formula Arts to turn the Shouki they destroy into energy bodies, putting them to various uses in daily life and beyond.

 Refers to the 26 Stellas who have each been granted one letter of the alphabet, A–Z—a “Code.” They are the apex tier among tens of thousands of Stellas, overwhelming all others in combat ability. Among the Misteltia Sisters, Lemulene is granted the code “M: Minerva,” Azerlete “H: Hecate,” Pianisia “A: Artemis,” and Calendula “F: Freya.”

 Refers to the four girls who make their home on “Misteltia Island,” at the center of the world map. They belong to the Hamlet “Misteltia” and live together, mainly under the guidance of their vice-captain, Arvel. Misteltia is given the role of the “Oversight Board” and is required to remain neutral; for that reason it must not intervene in quarrels between other Hamlets, or between Stellas. Fielding four Code Orders, it commands one of the largest fighting forces of all—and yet there are still upstart Hamlets foolish enough to pick a fight with the Misteltia Sisters. And each time, they end up getting blasted away as the Sisters’ tools for blowing off steam.

 A device through which a Stella manifests the power to fight. By channeling the wielder’s “ether” through the Famulus to construct a “Formula,” she can give rise to a “Phenomenon.” There are two ways of operating it: “Armed” and “Naked.”

 The combat mode in which the Famulus is turned into armor and worn about the body is called “Armed.” Because the armor handles stabilizing flight balance, generating an automatic defensive field, and semi-automating Formula construction, it lets even those who lack the “sense” to build and control Formulae attain combat ability at a practical, battle-ready level. This mode is used mainly by the Cavalry and low-ranking Stellas, or by high-ranking Stellas in situations where they wish to conserve their strength.
 Conversely, the mode of fighting bare—without materializing the armor—is called “Naked.” Because every part of Formula construction is performed with one’s own body and mind, it is overwhelmingly difficult to master. But for one who can wield it well, it draws out combat ability far more flexible and powerful than Armed could ever offer.