Neviril is an eighteen year old citizen of the advanced nation of Simulacrum. Her father is a prominent figure in the government and sends Sibylla priestesses on military missions.

Neviril enrolled as a Sibylla around the age of 7 alongside her best friend Paraietta, and quickly proved to be very talented. Her flying skills were the envy of many, and her leadership abilities as she aged were coveted as well. When she graduated her training she was assigned to Chor Tempest and became the piloting partner of a girl named Amuria. The two were romantically and physically involved, and became regarded as the best Simoun Sibylla duo in existence. Neviril was eventually promoted to Regina, becoming the leader of Chor Tempest. Unfortunately, Amuria was killed in the early stages of the war between Simulacrum and Argentum.

Neviril grieved the loss of her partner deeply and developed Post-Traumatic Stress Disorder. She had constant flashbacks of her time with Amuria and experienced vivid dreams of her death, to which Neviril would wake up from screaming and crying. Due to her lack of speaking, and even refusing to leave her bedroom, she was removed from her position as Regina. Dominura took over as Regina from that point onward.

The Simoun are mysterious flying machines that can only be piloted by Sibyllae, youth who haven't chosen a yet.

In the Simulacrum Theocracy, the Simoun are viewed as divine chariots; as such, the Sibyllae who pilot these holy vessels are also treated with reverence.

The Simoun can activate a magical power known as Ri-Mājon, which is meant to be used to write prayers in the sky for Tempus Spatium by inscribing enormous magical glyphs in the sky. Since some Ri-Mājon can destroy large numbers of enemy aircraft at once, they can be very useful in battle as well. Some Ri-Mājon patterns require more than one Simoun to collaborate in order to be drawn fast enough.

Simoun are two-seat aircraft with separate cockpits for the primary pilot ("auriga") and the navigator ("sagitta"), who is also in charge of weaponry and other accessories in combat. The auriga's cockpit is located in the forward part of the machine, and the sagitta's is situated behind and above, so that both have a clear view above and in front of them. A large green gemstone, instrumental for conjuring the Ri Mājons, and sometimes referred simply as the "Simoun Gem"[1] is located between the two cockpits, easily accessible by both pilots when the canopies are open.

Simoun are propelled by two helical motors (ヘリカル・モートリス, Herikaru mōtorisu), a technology monopolized by the state of Simulacrum[2], although even Simulacrum's technicians barely understand its principles. For example, one of their theories states that one rotor controls the Simoun's position in space, and the other affects the flow of time.[3] If needed, a Simoun can be equipped with large-caliber miniguns and/or special container pods for transporting and deploying a number of ground troops.[4]

When grounded, both Simoun's helical motors (which appear as large rotating wheels) align to a horizontal position relative to the ground, so that they can be easier accessed by pilots and maintenance technicians. During the take-off, however, the rotors align themselves vertically and remain so until landing ("helical motor tilt system").

Both Simoun and Simile have also got an deceleration and shock absorption system (inertia canceler) powered by the helical motors, since normally the Simoun are not equipped with seat belts or helmets for the pilots. The design of Simoun also denies the possibility of quick evacuation for pilots, since it does not foresee ejector seats or parachutes, however, if not critically hit and the altitude is low, a Simoun can pull off a soft emergency landing with the remaining helical motor lift (similar to autorotation principle in helicopters).

A Simoun requires two pilots to lift-off and steer in the air and, moreover, both of them must have not yet chosen their permanent (see the background for explanation), since having done so permanently disqualifies a person from further piloting. Because of its sacred status in Simulacrum, only young priestesses of the Tempus Spatium faith are recruited as Simoun pilots and granted the title Sibylla.

Although the Simoun are considered the most effective and maneuverable aircraft in the world, their main purpose lies within the Ri Mājons, since the only way to call forth their powers is by "drawing" certain patterns with the Simoun Gem in the air. This process is referred as "Praying to the Sky"[5] by the Sybillae and was initially thought to be their only purpose and goal as priestesses.

Before a Simoun can move, its Simoun Gem must be activated by a simultaneous kiss by both pilots, shortly after they have kissed one another.This detail has initially spawned the popularity of the series among the yuri fandom.] The routine take-off procedure includes activating the Simoun Gem beforehand, however, the Gem can as well be "charged up" in midair by this ritual. Apparently, all Gems of a particular Simoun unit or Chor (コール, kōru) are linked, since after the chor leader, the Regina (レギーナ, regīna), announces a Ri Mājon involving more than one Simoun, its pattern is immediately displayed within each Simoun Gem of the chor as necessary. Notably, aside from the instant Ri Mājon pattern transmission via Simoun Gems, the Simoun do not have any other means of wireless communication, unlike Argentine heavy fighters, for example. The Simoun must therefore use an extendable communication wire with a magnet (or a suction cup, which is not clear) at the other end that, after having been ejected from a Simoun's stub "wings", can attach itself to another Simoun and transmit sounds from their cockpits in both directions. After the communication session is over, the wire is pulled back inside the Simoun.