Fully-sentient Gear from Guilty Gear. She was tired of humans pushing around Gears and turned them against them, starting the 100-year war known as the Crusades. The war ended with the victory of the humans when the Sacred Order of Holy Knights, a human resistance, sealed Justice away causing all Gears to cease to function.
Eventually, Justice was revived by Testament only to be defeated and destroyed by the winner of the tournament in Guilty Gear, which, officially, was Sol Badguy. Since her death in Guilty Gear, she appears in its sequels as a secret unlockable character (in Guilty Gear XX Accent Core, she, along with Kliff, are absent, but in Guilty Gear XX Accent Core Plus, she is unlocked from the beginning). She is also Dizzy's mother.
An interesting fact to note is that Justice may actually be Aria, as Valentine, who is an Aria copy, has another form that resembles Justice (save for the color scheme).