Guilty Gear Xtra (photo)
Aria was Frederick's lover who had lived more than a hundred years before the main events of Guilty Gear. She was one of the key scientists of the Gear Project along with That Man and Frederick. She was a genius who graduated and got her PhD in her teens, and was two years younger than her lover. She was described as mature for her age, yet showed shades of youthfulness.
Little is known about Aria aside from the descriptions found in That Man's story, and she is presumed to be deceased before the events of the main story. However, Valentine is shown to be exactly physically identical to her and has parts of her memories. It is also heavily hinted that Aria is, in fact, Justice, since the Gear's last line was "I hope the three of us can talk one last time". That Man also told Sol in Guilty Gear 2 that both of them killed her.
Aria's name may also be a homage to the Russian Heavy Metal band ARIA (Ария). Since Guilty Gear has many musical tie-ins for each of the games.