Mirage is Aria's special ability. It is an illusory ability.
The ability has three tiers. Its usage is limited by a bar dubbed the Focus Gauge, which each tier will deplete a certain amount of. In addition, if the illusion is touched, or the wielder of the ability is harmed than it is dissipated. The wielder's true form can also be seen by reflections, or certain people able to resist illusory-type abilities. It can also only work on fully corporeal beings, with non-corporeal beings being unable to be affected by Mirage, and half-corporeal beings such as Ludwig looking highly strange when mirage is used on them.
The first tier is Beguile, which requires only one-eighth of the focus gauge to be used. This is the first illusory ability tier Aria displays in the story. She nigh-constantly uses it to disguise herself as a monster in order to live in Red Tulip town. The ability can gives Aria a horn and a pair of red eyes, which she can also apply to others. (She does so to Nino.) In addition, Beguile makes Five look like a normal human girl, with the modification of dark purple eyes and no horn.
The second tier is Facade, which requires half the focus gauge to be used. Facade allows Aria to change her appearance as a whole, appearing as a completely different person. She uses Facade to fool Solis, and to escape from the Jail in chapter 2. It is implied that facade makes the recipent look like a person who already exists in the world. It seems that Aria was somehow able to disguise herself as a cell wall and blend in with the wall, while her mysterious savior somehow deposited a file through the window and disappeared.
The third tier is Fata Morgana, which requires full focus to be used. It is unknown what this tier does.