Jack-O' is an artificial lifeform created by Asuka R. Kreutz to prevent the revival of Justice, the herald of destruction. Because Justice was built using Sol's former love Aria as a base, Jack-O' stopped Justice's awakening by fusing with Aria's soul, which allowed Aria's soul to fully restore within her — though Aria's consciousness has not yet manifested, leaving Jack-O' in an unstable state. Her personality remains unchanged, and while she maintains a composed demeanor, strong emotions cause her to revert to childlike behavior. She wears a special mask during battle specifically to hold that composure in place.