Ouka is an enforcer of Kato in the video game Shadow Hearts: Covenant. She is the last of the Mutant Apes, a group of genetically enhanced super-soldiers created from fallen members of the Japanese Military. She is reconstituted from the body of Yoshiko Kawashima.
The origins of Ouka go back quite a bit as they both reference the previous game Shadow Hearts and Kato's actions in Shadow Hearts: Covenant are entirely motivated by his desire to create Ouka from the very beginning of the game's introduction.
Kato was officially made Lt. Col. Kawashima's second in command during her activities in Shanghai but in truth was sent to spy on Lt. Col. Kawashima for Minister Ishimura, the shadow ruler of Japan. Kawashima won over Kato's admiration and loyalty and he became genuinely devoted to Lt. Col Kawashima, as a result Kato began avoiding certain details or just plain falsifying his reports back to Ishimura in favor of giving his commander more leeway to pursue her own ambitions in Shanghai. Eventually Kato's sketchy reports prompted Ishimura to send a military inquest group which was prepared to assassinate Kawashima. With the help of Yuri Hyuga and his friends, Alice Elliot, Zhuzhen Li and Margarete Zella, Kawashima was saved from the assassination attempt. However late in the game it is revealed that after Yuri and his friends left Shanghai that there was a second incident with the Japanese military: Kato had just confessed both that he was a spy planted in Kawashima's service and that he had grown to love her when a messenger envoy from Japan arrived with a priority message for Kawashima. The moment Kawashima left the room she was shot in the back, the messengers were just less conspicuous assassins sent by Minister Ishimura.
Kato blamed himself for the entire affair thinking if he had done his job instead of breaking the rules for Kawashima she would still have been alive. Kato continued to serve Ishimura and was rewarded for his loyalty by being promoted to the official diplomatic envoy of Japan. Kato re-encountered Yuri while touring Europe. The two catch up early in the game Shadow Hearts: Covenant and Kato tells Yuri about his new position and finds out Yuri too had lost his love when he learned that Alice had died. Yuri mentioned that he was after a terrorist group called Sapientes Gladio that he was hunting down. Kato wished him luck and said he would tell Yuri anything if he heard about them, but in truth Kato's tour of Europe was a cover for enlisting them as allies to Japan in their campaign to conquer Europe. In exchange for Kato pledging Japan's support to them Saspienties Gladio was to acquire a book called the Émigré Manuscript for Japan. After causing much chaos in Europe and stealing the Émigré Manuscript from Yuri the second in command of Sapientes Gladio, Nicolai Conrad gave Kato the book. The Émigré Manuscript was what Kato had personally wanted from the very beginning for it had the power to bring the dead back to life, although usually there were complications to such resurrections.
Still Kato had convinced Isihmura that the book could be used to create undead super soldiers for Japan and the program was to be called the Mutant Apes. Ishimura knew Kato just wanted to bring Kawashima back to life as a Mutant Ape but humored the notion since it meant he would get a small army of undead super-soldiers once the process was perfected. After saving Nicolai, as the sole surviving member of Sapientes Gladio, Kato returned to Japan. He gave Nicolai to one Dr. Hojo to exorcise the demonic powers he had come to possess and immediately reported in to Ishimura then went to put the Émigré Manuscript to use. Kato knew the follies of attempting complete resurrection with the Émigré Manuscript so instead of trying to bring back Kawashima as she was Kato used a piece of DNA the Military kept on hand and used it to clone Kawashima and only used the Émigré Manuscript to speed up the aging and psychological development process. So the first Mutant Ape to be revived with the Émigré Manuscript was created and Ouka was born.
Ouka awoke in a lab and heard Kato over a loudspeaker. Kato told Ouka her name and said she was to get through an obstacle-course in order to meet him. The course was laced with accuracy training exercises that needed to be cleared to unlock the doors. On her way there Ouka met Hien and then Raiden both of which requested she prove her strength to them by fighting them before joining her in completion of the course. Hien and Raiden were Mutant Apes that predated Ouka however they had been revived using radical medical treatment and cybernetic implants. Ouka, Hien and Raiden completed the course and beat the mecha-tank waiting for them at the end. Once the course was completed Kato appeared and congratulated them, Kato made sure Ouka was adapting to her body and then told her that she and the others existed to serve him.
During Hojo's experiments in attempting to extract the demonic powers of the Demon God Astaroth a monster called Skoki was accidentally created. Hien and Raiden were sent to contain it without killing it however holding themselves back made it difficult. Ouka used her powers to save Raiden and Hien but the creature demonstrated it's power to teleport and got away. Kato hit Ouka for disobeying his orders and saving Hien and Raiden and then stated the creature needed to be caught. Kato emphasized that Shoki was to be found and that they were not to risk fighting it but just follow it, tell him when they found it and observe it's fighting style while they waited for him to arrive and give them further orders.
The Mutant Apes did as they were told finding Shoki on the streets of Yokohama. Ishimura's elite men the Iron Soldiers were attempting to kill Shoki but being slaughtered. Kato arrived but noticed that Yuri and his friends were about to fight Shoki. Kato ordered Ouka and the others to get ready to fight and went to meet Yuri as before he fought the monster. Yuri's attention went from Shoki to Ouka, the second Yuri saw Ouka he became very upset, realizing what Kato had done and who had the Émigré Manuscript. Kato refocused Yuri and told him that his Mutant Apes were just there to help and would give him an an opening. Though still upset Yuri agreed to join forces and Kato ordered Ouka and the others to attack the Shoki's monstrous spawn to give Yuri and his friends a chance at the monster it-self. After providing support fire Ouka and the others reported back to Kato to observe them at a safe distance, Kato told them to watch carefully as Yuri and his friends were their ultimate opponents. When Yuri and the other heroes beat the Shoki Kato thanks them for their help. Yuri notes that he sensed Nicolai in that monster. Kato tells Yuri the creature was accidentally created while attempting to extract Nicolai's power. In gratitude Kato told Yuri where Nicolai was being held but cautioned him that once he arrived there they would throw all the forces at him and his friends, a notion Yuri was unconcerned with.
Ouka, Hien and Raiden returned to the Hojo research facility with their master. Dr. Hojo did not want to risk having to wait for months for a third shot at waiting for Nicolai's power to peak so he opted to use full power of his machine regardless of the risk to Nicolai. Kato attempted to stop the mad doctor but failed and Dr. Hojo had Nicolai's ego completely subverted to bring out Astaroth. Yuri and his friends were there for the experiment and saw as it went awry. Astaroth was not brought out and contained but rather just seized complete control of Nicolai's body. Once brought-out Astaroth broke through his restraints. Yuri and his friends confronted Astaroth but as Ouka, Hien and Raiden were dispatched to deal with him the demon lord teleported away talking about destroying the world. Kato told the Mutant Apes Astaroth was their responsibility and told Yuri they would find a way to stop him then all four teleported away themselves to find Astaroth.
The Mutant Apes would find Astaroth in Mount Fuji, Astaroth was there to send his power through the ley-ines of Mt. Fuji, destroying Japan and throwing most of the planet into ecological chaos. While Hien and Raiden were covering their path Ouka and Kato were discussing the future. Ouka told Kato she had feelings for him. Kato knew Ouka was not really Lt. Col. Kawashima but had similar feelings for her and that once they had dealt with Astaroth they could work out their relationship as something more than master and servant. As Hien and Raiden arrived Ouka noticed a lava flow to a protected cavern and the lava parted, Kato said they were being invited in by Astaroth. Ouka and the others fought Astaroth but were unable to beat him. Kato was badly beaten and Hien and Raiden were both injured as well but they were abe to survive long enough for Yuri and his friends to arrive. Hien warned Yuri not to get close to Astaorth's claws in particular since their were poisoned with Malice one scratch would kill him, Yuri thanked Hien for the warning and then went to defeat Astaroth once and for all. Yuri and his friends beat Astaroth and returned Nicolai to his normal-self, however Nicolai was enraged that his original plans had gone so far off track and took out his rage on Kato for using him. Nicolai picked up a chipped off piece of one of Astaroth's nails and struck at Kato with it, but Kato was blocked by Ouka who had gotten in the way to save him. Ouka died in Kato's arms professing her love for him. Ouka's death triggers Kato's anger and hatred and he promptly picks up Nicolai by the head and crushes his skull with his bare hand afterwards Kato, Hien and Raiden leave with Ouka's body.
Kato met with Yuri later the graveyard where Lt. Col Kawashima was buried. Kato had buried Ouka in a small unmarked grave next to Kawashima's headstone. Kato's decided to sever his ties with Ishimura and decided he was going to undo time with what he learned from the Émigré Manuscript but gave the book back to Yuri, telling him to use it if he wanted to stop him. Kato also gave Yuri one final parting gift before leaving, he told him where Minister Ishimura was, knowing Yuri would go there to avenge Lt. Col Kawashima for him.
Ouka's ghost reappears at the end of the game when Kato transforms into Susano-o in the game's final battle. As a ghost she transforms into the Yata Mirror, in this state she has the new ability to shoot beams of spiritual energy and enhance her allies.
Ouka is completely devoted to Kato, as are all the Mutant Apes, but in addition to her loyalty Ouka seems to have romantic feelings for Kato as well. Ouka seems quiet and reserved, aside from her love for Kato she seems not to have any outward emotions. Ouka worries that because she is not really Yoshiko Kawashima, Kato will not love her but upon speaking to him about it finds that while he is aware of the fact he still had the feelings for her. Kawashima never had a chance to reciprocate Kato's feelings so he may have been just re-projecting onto Ouka as a version of Kawashima that would love him. Unfortunately this concept is never given a chance to be explored in light of Ouka's sacrifice.