She wears sandals and has bandages wrapped around her lower legs, and cloth bracers around her forearms. Her typical expression is one of detached hopelessness, seeming to never smile or laugh, even when she thinks she has succeeded in escaping the island with Shiren.
Koza is a meek yet serious girl who can't stand up for herself and she is very submissive as a result of living for so long under Mukuro's abusive rule. However, Koza is ultimately shown to be a master manipulator, successfully fooling everyone around her and even convincing them to kill one another in a complex, multi-step plan she had concocted as a way of achieving her main goal of escaping the prison island.
After a lifetime of living there, which she describes as “hell”, she had been repeatedly abused by many different people on the island, and learned to survive not by fighting back physically, but instead manipulating people into destroying each other and themselves through careful psychological strategies aimed to benefit her.
Koza was born in the Ryukyu Islands. It is unknown who her father was, however Koza mentions that her mother was killed by Mukuro. Koza, Mukuro, and Mugen all grew up on the same prison island together, but the three don’t seem to be related by blood.
As they grew up, Koza developed feelings for Mugen. One night, Koza talks to Mugen about the stars in the sky above them, and Mugen seems to listen.
After Mugen is captured by government officials following a failed robbery of a merchant ship which Mukuro successfully escaped from, Koza and Mukuro watch as Mugen is cornered by officials on the edge of a cliff with his hands tied, about to be executed. Koza panics and tries to run to Mugen to help him, but Mukuro holds her back, and she witnesses Mugen jumping off the cliff voluntarily to prevent the officials from being the ones to kill him. When Mugen does not reappear on the island afterward, she assumes that he is dead and mourns the loss of him.
One day, Koza appears to be despondently playing in the sand on the beach, watching as it slowly falls through her fingers like an hourglass, when she's noticed by Jin, Fuu and Mugen. Koza is relieved and overjoyed to see that Mugen is alive, but they are ambushed by Mukuro. After Mugen convinces Mukuro to let them go, Fuu talks to Koza, and Koza gives Fuu advice on how to get to Nagasaki.
Koza tries talking to Mugen in an effort to get him to open up about what happened after he jumped off the cliff, but Mugen brushes her off and leaves, lying by saying that he doesn't remember her talking about the stars. Later, Mukuro tells Koza of his plan to rob a ship carrying a large amount of money (about 60,000 ryos) with Mugen, and letting Mugen die in the resulting fight to sever loose ends. Despite her strong feelings towards Mugen, she obeys Mukuro and does not tell Mugen about the secret plan, and witnesses the boat Mugen was on exploding.
In Misguided Miscreants (Part 2) Koza appears in Mugen's flashbacks, and in the present day on the port, Koza asks Jin if he will kill Mukuro to avenge Mugen. Meanwhile, Mukuro and his associate Shiren (the mole he had planted on the government transport ship) are discussing matters in a hidden area that they plan to use until it’s safe to escape, and Mukuro comments on his efforts to hook up Shiren with Koza, implying that Koza had refused Shiren before and was being forced by Mukuro to let Shiren do what he wanted with her.
Jin and Koza roam through the nearby village until a group of investigators show up to ask them about the recent robbery, stopping when another individual manages to locate the ship but finds the money missing.
Later, Mukuro brings Koza to a shack where Shiren is waiting for her inside, and instructs her to go in and presumably become intimate with Shiren. Koza appears unwilling to do so, but ultimately enters the shack alone, and as soon as the door closes behind her, Jin shows up to confront Mukuro, killing him in only two strikes. He goes to check on Koza, who by then had snuck out the back of the shed they were inside of with Shiren, and Jin realizes that she had manipulated him into killing Mukuro for her, and likely did the same to Mugen.
As the sun sets, Koza and Shiren (who are now both much more openly affectionate with one another, in sharp contrast to their previously cold behavior) begin making their way to the spot where the stolen gold was stashed and unexpectedly encounter the still-limping Mugen on the beach. Shiren charges forth to attack, but Mugen does away with him instantaneously. He continues to limp towards a terrified Koza, who braces herself to be cut down as well. But out of disgust, and perhaps as a way of punishing her for her treachery, Mugen merely continues limping past her, not even stopping to acknowledge her presence. Koza then falls to her knees and begs for him to please kill her, and to not leave her by herself on the island she had so desperately sought to escape from all of her life. But he ignores her pleas, leaving her sobbing on the beach, alone.
The arc ends with no one knowing where the stolen gold was hidden, as only Mukuro and Shiren knew its secret location, and both were killed before disclosing it to anyone. It is unknown what happens to Koza after the main trio leave the island without her.
- It is unclear if Mugen knew Koza's feelings for him.
- It is unlikely that Koza and Mukoro are actually siblings (even though they have the same skin tone) because Koza stated that Mukoro killed her mother, not our mother.