Casey Okada is one of the co-leaders of the Renegades and one of the primary antagonists and final bosses in Double Dragon IV, along with her twin sister Shannon. Both entrepreneur sisters run the Okada Company, which rules over Japan. Their family was slaughtered for being weak and now Casey is obsessed with being strong. Unlike most other Double Dragon characters, she uses a gun at times.
The Okada Sisters, Casey and Shannon, are the secret real bosses behind the Renegades. They are a couple of twin high profile entrepreneurs and owners of the Okada Company, which has taken over Japan. It is later revealed that their family was slaughtered for being weak and now Casey is obsessed with strength.
Looking to expand their territory to America, the twins order Jake, their lieutenant in those shores, to take over the West Coast; a task that proves easy to perform as the Black Warriors, a criminal syndicate that was in power at the time, had recently been defeated by the Lee brothers, Billy and Jimmy. Jake has the gang's former leader, Willy Mackey, assassinated and then recruits the remnants of his forces.
Aware that the brothers could be a potential threat to their plans, the sisters have Marian, Billy's girlfriend, kidnapped and brought to Japan in order to lure them out of the way.
The Lee brothers go to Japan and fight to the top of their tower, although Casey manages to escape in a helicopter. The brothers beat information out of Rowper to find them. Ashida takes the brothers to the sisters' private villa after being defeated by them. Once there, the sisters drop the brothers down a trapdoor into the basement. They fight their way out and save Marian, who tells them the sisters went to the "Bridge of the Gods".
The brothers go after them and finally beat them down. Defeated, Casey urges them to finish her off, but they tell her that they don't kill. Regretful, she wonders if she can go on living after all she had done, but Shannon comforts her by telling her she is the strongest woman she has even known. The brothers confess her that they have weaknesses too, but use their martial arts to overcome them. Hearing this, Casey wonders if she can start over, to which the brothers respond that she is already on the right path.