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 teen and was two years younger than Frederick. She specialized in the field of cytology and Artificial Intelligence. She was described as mature for her age, yet showed shades of youthfulness. It was later discovered that she was infected with Treponema Pallidum(TP) since birth.
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 may have been 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. In Guilty Gear Xrd -SIGN-, it is revealed in the Guilty Gear Xrd Library that Justice was a Copy created using Aria's cryogenic DNA as a genetic template. Jack-O' Valentine was also directly synthesized from a hasty memory core dump of Justice's copied memories of Aria, which left her mind and memories fragmented and unstable.
- Like Elphelt and Ramlethal, Aria has a strong dislike towards bugs, particularly cockroaches.
- In That Man's short story as well as her brief appearance in a photo in the manga, Aria has long red hair, much like Justice, but decided to cut it short to attract Frederick's attention.
- One of her Valentine copies, Elphelt, besides having a similar appearance, also seems to exhibit aspects of her personality.
- Aria's name most likely comes from "aria", a vocal solo part of a larger work (with the typical context being operas).