Seriously, this guy is scary at first.. but his ending is really sweet. Koukin is also known as Zhou Yu. If you have any questions about a certain part of the story in Gentoku and Chuubou’s routes, the answer lies in Koukin’s route.
Koukin is the main strategist of Chuubou’s army who often guides the young ruler. He already worked for Wu ever since Chuubou’s father was still alive and was best friends with Hakufu, Chuubou’s older brother. He always puts on a smile and it’s hard to see what he really feels since he never showed it. He’s good at playing biwa, but Hana thinks that the melody he plays are cold and lonely.
While he seems to be friendly, Koukin didn’t trust Hana’s skill as a strategist. He tested Hana’s ability by ordering her to gather 100,000 arrows in 10 days, which she accomplished by using what actually was Koumei’s strategy in history.
Despite Hana initially giving them strategies to minimize casualties, Koukin planned to burn and destroy Moutoku’s army in the Battle of Red Cliffs. Hana, being the kind soul that she is, said that they shouldn’t cause unnecessary death. Koukin only replied that she should stop playing a saint since he hates that kind of person the most.
Koukin later told Hana that during a meeting, Gentoku agreed that she should stay with Koukin’s army for sake of the alliance. He then returned to Kei with Shikei and Hana, taking her further away from where Gentoku was. Soon Hana also found out about his plan to give Keishuu to Chuubou after taking it instead of giving it back to Gentoku.
When he delivered Hana’s book from Unchou, the book glowed and they were thrown back in time to the Yellow Turban Rebellion period. Knowing that she had to tell him the truth, Hana eventually told him about the book and her origins. Koukin didn’t believe such an illogical explanation at first, so he decided to go to Rakuyou in order to see if she’s telling the truth.
They met Son Bundai, Chuubou’s father, on the way and was offered shelter in their camp. During their stay with Bundai’s troops, Koukin hid his identity and asked Hana to called him Kin instead of his real name.
In order to go back to the original time, Koukin wanted to help the rebellion succeed. Even though it would take them to the future when Bundai and Hakufu died in battle.
Koukin then told Hana about Hakufu’s death. That time they had an argument and went on separate ways. After cooling his head off and thinking about it calmly, Koukin wanted to talk to Hakufu again. However, Hakufu was assassinated by a poisonous arrow and that chance to talk was gone forever. Because of Hakufu’s death, Koukin felt guilty and wanted to unify the lands under Chuubou’s power. For Hakufu’s sake, since it was his goal when he was still alive.
Soon, Hana and Koukin helped Bundai take Enjou. On the other hand, they also helped the rebellion to flee without causing casualties on both parties. Koukin had to stay with the rebellion to guarantee that Hana was telling them the truth, and she went looking for him during the chaos. He got angry as she puts herself in danger by doing that, but Hana said it was because she’s worried about him.
When they were escaping, the book suddenly took them back to their castle in Kei. Happy that they managed to come back safely, Hana hugged Koukin. He was about to hug her back when he realized what he was about to do. He restrained his hands from hugging her and told her to move away.
Hana found out from the book that Koukin will get shot and eventually die during the next battle. She asked to be allowed to go with Koukin when he departs to attack Kouryou. Koukin didn’t want to bring her to the battlefield, but he eventually lost to her persistence.
Worried about Koukin’s fate, Hana gave him a strategy from the book for attacking Iryou. However she later found out that it would also result in the same future, his death. Hana did all that she can to prevent this from happening, but eventually Koukin was shot by a poisoned arrow when Hana came into a battlefield looking for him.
Hana felt guilty because she didn’t return to the camp even though Koukin told her to do so. That night she asked the doctor if she could come inside to see him, and she watched Koukin crying in his sleep apologizing to Hakufu. Hana held his hand and said she wants to know more about him. She wants help him, and she begged him not to die.
In his sleep, Koukin heard Hana’s voice telling him not to suffer and cry anymore since everything would be okay.
That night, they were ambushed in the camp. Koukin got up despite his condition and Hana’s worries. Instead of returning to Saisou he attacked the castle instead and won the battle.
Even after returning to their castle in Kei, the book still says that Koukin will eventually die. He said that his wound was already healed, but Hana was still worried about him because of what the book said. Seeing how worried Shoukyou was about Koukin, Hana remembered what Shoukou told her when she first arrived in Kei: the subject of them getting married. This leads Hana to think that Koukin and Shoukyou were engaged.
One night when she asked him to treasure himself more, Koukin answered that being with Hana discomforted him and they shouldn’t be concerned about each other. For their own sake.
Later on Hana figured out that Koukin was planning about marrying Gentoku to a fake Shoukou and have him assassinated later. She tried to talk to Koukin about that, but his room was tightly guarded. With the help of the real Shoukou and the Kyou sisters, Hana eventually got to talk to him. Koukin finally revealed his real motive to her: he was trying to destroy all the other forces in order to bring unification under the Son family, including both Moutoku and Gentoku.
Hana told him that what he did was only to cover the wound and regret he got from Hakufu’s death. He was running away from the pain caused by his past. Hearing this, Koukin took out his sword to attack her.
With his sword at her neck, Hana asked if killing her would save him from all his sufferings. If it does he’s welcomed to kill her. Koukin asked why she said such things, and Hana answered it’s because she loves him, she wanted him to live. Koukin was surprised to hear her confession. In the end he dropped his sword, saying that he should have her killed earlier.. before he couldn’t bring himself to do it. He collapsed soon after, due to the poison from his earlier wound.
Holding his hand, Hana stayed with him until he regained consciousness. Koukin finally admitted that what Hana said was right. He felt that until Hakufu’s dream of unification is fulfilled, he won’t be forgiven for letting Hakufu die. After Hana said that it’s normal to feel sad since Hakufu was a precious person to him, Koukin said that in his dreams he heard a forgiving voice. And that voice was hers.
The marriage plan between Gentoku and Shoukou was put on a halt for now, and Hana was thinking about returning to Gentoku’s army and to her world. Due to an intentional misunderstanding caused by Shoukyou, Hana end up seeing Koukin in his room. Koukin asked if she came because she’s worried about him. After their last conversation Hana didn’t come to see him again, and he thought that she might already forgot him.
When she told him that she already found the way home, he asked if she really doesn’t have a reason to stay in this world. Since she doesn’t seem to understand, Koukin asked her if she loves him, confirming her earlier confession. Remembering the things she said, Hana flustered and said that Koukin already had Shoukyou. Koukin explained that it was only a rumor created by people, since in reality the Kyou sisters and him were only playmates — along with Hakufu when he was still alive.
Koukin asked her if she still wants to go home after hearing that. Hana responded by asking him why does he say such things, since he didn’t think anything of her. Koukin answered that he loves her, though initially he didn’t trust her as Koumei’s apprentice and was often irritated because of what she did. When he heard about her going back home, he didn’t want to lose her. Koukin then asked her to take responsibility.
Thinking about how she loves him and wants to stay by his side, Hana asked him to promise her that he won’t put his life in danger and face death anymore. Koukin promised that he won’t ever say those things again, and asked her to stay with him. He kissed her after that.
During a banquet party in the epilogue, the Kyou sisters talked about it was irritating to see Koukin and Hana not making a move even though it was clear that they love each other. Koukin asked them to stop talking about that, but they didn’t want to stop teasing him. When Shoukou came to get the sisters away from them, they talked about her upcoming wedding with Gentoku.
Hana said that Gentoku will definitely make Shoukou happy, which made Koukin jealous because she thinks Gentoku is a great person. He denied that he was jealous even though Hana didn’t say anything, which only proved that he was jealous. She found that side of him cute.
After teasing him about his relationship with Hana, Chuubou and the people around him asked Koukin to play his biwa. Hana also said that she wants to hear his biwa, and so he played a song. She noticed that the melody he played doesn’t sound cold and lonely anymore. Its gentleness reflecting his current heart.