Airan Jo is a human female from Earth 2. She features as a protagonist and playable character in the first Sin and Punishment game. Much like her male counterpart, Saki, Airan is a member of the Savior Group, no doubt revived and modified by Achi following a death at some point before the game starts.
At the beginning of the game, she accompanies Saki and Achi on their desperate attempt to pierce through the Armed Volunteer forces and board a transport to leave the city. After arriving at the transport, Airan only has enough time to reach the controls before Radon, a human transmogrified into a Ruffian, ambushes Saki.
Saki defeats both Radon and Kachua, the commander of the Armed Volunteers in Tokyo. However, a tsunami of blood from the felled enemies engulfs Saki, turning him into a Ruffian. Recognizing that he is in a berserk state, Achi teleports Airan and herself away from Saki, just before he destroys the transport.
Teleported to the Armed Voiunteers fleet (the safest location), a transmission from the leader of the Armed Volunteers (Brad) reveals that the fleet is moving to attack Saki. Resolved to protect Saki and return him to human form, Airan springs into action, blasting her way through the traps and caught ruffians between her and Brad.
Following a confrontation with Brad, Achi uses telekinesis to rip a section of floor out of the carrier and levitate it around the fleet, allowing Airan to destroy large portions of the Armed Volunteers. In a final conflict with Brad, she defeats him. As the Armed Volunteers plummet to their doom, Brad self-destructs, rather than turn himself into a ruffian.
A giant airship appears above the destroyed fleet, and Achi takes Airan there to once again cause massive amounts of damage in an attempt to protect Saki. After chasing down a projectile fired by the large airship, Achi tells Airan that she will have to shoot Saki in the head so that he can be cut open to have Airan placed into his heart, communicating directly through from her mind into his soul.
Airan refuses to shoot, thinking that Achi is lying. In response, Achi sends Airan into an illusionary world set ten years in the future, accompanied by a child named Isa. Fighting through the ruffians that have infested the train they are on, Airan learns that they are in Long Island, circa 2017. Furthermore, the child's full name is Isa Amamiya. Implicitly understanding that Isa is her child, she confronts the Saki of the illusion, who is accompanied by ruffians and appears to have betrayed humanity. Covering Isa's eyes, Airan shoots the illusionary Saki, resolved to restore his humanity in reality and prevent the events she has seen.
The illusion is broken, and Airan realizes that she has shot the transformed Saki in real life, knocking him out and allowing Achi to place Airan inside. Waking up on a beach, she finds that Saki has returned to a quasi-human form, still not entirely accepting of his humanity. As the ruffians attack, the pair seek shelter for the night.
Achi kidnaps Airan, rendering her unconscious, and Saki storms the Ruffian nest where she is being held. Following a climactic struggle with Achi, Saki is victorious. The two wonder about their future, as Airan notes that Japan is in ruins. When Saki wonders if the police would accept him, Airan supports him wholeheartedly. First, however, they decide to eliminate the Ruffians as a team. The credits roll, featuring Airan enclosed within Saki as he marches onward in his larger form.
At some point, their son Isa is born. Neither Airan nor Saki are featured in the second game, and no concrete information exists on their fate.