A Magical Girl (魔法少女 Mahou Shoujo) is a human female granted magical powers and one Wish by an Incubator, in exchange for killing Demons and extracting Grief Cubes, in some way staving off the thermodynamic heat death of the universe.
Magical girls are also called Puella Magi, based on the title of the anime from which To the Stars derives, Puella Magi Madoka Magica, but this term is not used within the anime or To the Stars. The term derives from a poor latin translation of "Magical Girl", though it is possible that it is actually a translation of "Slave of the Magi".
A magical girl is first created when a human with potential, invariably a female between the ages of 9-20, is offered what is termed a Contract by an Incubator. The girl makes a wish, performing a one-time reality distortion of their choice, while the Incubator simultaneously extracts their soul, encasing it in a gem called a Soul Gem. The girl acquires a range of magical powers, but the use of these powers imposes a cost in the form of corruption to their soul gem, which appears as a blackness within the light emitted by the gem. In addition, merely maintaining their body imposes a small constant drain on the gem. Should the gem ever become fully corrupted, it will crack, causing the magical girl and her gem to disappear, effectively killing the girl.
The only way to reverse soul gem corruption is by cleansing it with an unused grief cube, acquired by destroyed a demon in combat. After use, the grief cube is saturated, at which point it is given to an Incubator.
The overall process somehow lowers the entropy of the universe, but it is unclear how this process occurs. What is clear is that the continued survival of the magical girl via the defeat of demons and use of grief cubes is necessary to the process.
It is unknown what causes some humans to have potential while others do not, nor why potential is restricted by gender and age. Only the Incubators seem to be able to reliably determine who has potential. Notably, girls with potential can use an Incubator as relay to telepathically communicate with each other, and may telepathically communicate freely with contracted magical girls. This is the only known possible pre-contract use of magic.
When a magical girl dies for any reason, the Goddess comes to collect their soul and take them to the afterlife, though this process is unobservable from within the material universe.
 Previous System
With the exception of death due to complete soul gem corruption, or due to physical destruction of the soul gem, magical girls are clinically immortal, immune to both aging and sickness. While it is natural to assume that the magical girl is the human body that is their vessel, the girl is in fact the soul gem itself. Moving the soul gem 100 meters or more from the body causes the loss of the consciousness, while the body dies on the spot, becoming a corpse. Returning the soul gem revives the body. It is also possible for the soul gem to occupy another body, such as the "blank slate" clones the MSY generates as backup bodies for its members. While the soul gem appears to contain the core sentience of the magical girl, the sentience does not appear to function without a suitable substrate for consciousness and, in the absence of any other options, the gem may attempt to reconstruct a body de novo.
While occupying a suitable body, a magical girl is capable of prodigious superhuman feats, including the exhibition of strength, speed, resilience, and reaction time far beyond the human norm. By performing a transformation, wherein the girl adopts a costume that replaces their clothing, the magical girl enters a form capable of much more open exhibitions of magic. The exact type of magic that can be used varies heavily, depending partially on the wish that the girl made. In addition, the vast majority of magical girls also manifest a primary weapon while transforming, which is often heavily used in combat.
Magical girls are also capable of manipulating their bodies significantly, changing their physical age rapidly, modifying physical appearance, and correcting any pre-existing defects. It is not known if there is a limit to this manipulation.
 Classes of Magic
 Magic Development
 Costume Choice
See also: Mahou Shoujo Youkai
Until the advent of the Contact War, magical girls and Incubators concealed themselves from the attention of the world as a whole, operating without the knowledge of official authorities or society. Great effort was expended in maintaining this masquerade, for reasons that are unclear. Only when the Mahou Shoujo Youkai (MSY) ordered a counterattack against the Cephalopods at the Battle of Epsilon Eridani was the masquerade abandoned.
Until the early twenty-first century, the vast majority of magical girls were short-lived despite their clinical immortality, succumbing early in their lives to death in combat or soul gem exhaustion. Nearly all magical girls were self-organized into close knit teams which hunted demons together, usually staking out a claim to a physical territory and defending it from other teams. Generally, interactions between teams were anarchistic, with little in the way higher level social organization or mores. Notable exceptions include the first Système Magique Cordial, numerous despotic cities with teams subservient to a one or a couple of powerful teams, and a few Neutral Zones which operated as economic and trading centers.
The mid twenty-first century saw the rise of the MSY, a powerful magical girl shadow government whose reorganization of demon hunting and extensive corporate operations ensured both grief cube and financial stability to its citizens for the first time. Life expectancy soared, and increased leisure time enabled the majority of magical girls to withdraw from full-time demon hunting, focusing on other interests and pursuits. After a century of competition with rival organizations, the MSY came to encompass all living magical girls, with the official endorsement of the Incubators.