In fanfiction, their relationship can develop through trust - building. Yusuke has to prove to Keiko that he can control his demon side and not let it harm her. Keiko, on the other hand, has to trust that Yusuke is still the same person she fell in love with despite his new form. They can also have moments of vulnerability, like when Yusuke is struggling with his identity as a demon and Keiko is there to comfort him, which cements their relationship further.
The relationship can be developed through shared experiences. For example, if they face a common enemy together. Yusuke's demon powers can be used to protect Keiko, which deepens their bond. Also, communication is key. Yusuke can open up to Keiko about his new life as a demon, and she can share her fears and concerns about his transformation.
The relationship could develop in a rather unique way. Maybe Yusuke saves Keiko from a life - threatening situation, which makes Keiko see him in a new light. Yusuke, on the other hand, is drawn to Keiko's purity and kindness. As they spend more time together, Yusuke starts to change for the better because of Keiko's influence. Their relationship might be full of misunderstandings at first, but as they communicate more, they grow closer and closer, eventually falling in love.
Yusuke would likely become more withdrawn. He was always a bit of a rough - around - the - edges character, but Keiko's death would make him more somber. He might lose his sense of humor that he sometimes showed around her.
In Yu Yu Hakusho lemon fanfiction, the relationship between Yusuke and Keiko can be quite diverse. Sometimes it delves deeper into their emotional connection, showing a more passionate side of their bond that might not be as fully explored in the original series. It could involve scenarios where they face new challenges together and their relationship grows stronger through those experiences.
Yusuke might cope by throwing himself into his work as a spirit detective. He could try to distract himself from the pain of losing Keiko by taking on more and more dangerous cases. This would be his way of running away from his emotions at first.
One idea could be that Demon Yusuke has to protect Keiko from other malevolent demons while hiding his true nature from her at first. As the story progresses, Keiko discovers his secret and has to come to terms with it. Another plot could be a time - travel scenario where Yusuke goes back in time to save Keiko from a danger that she faced in the past, which changes the course of their relationship in the present.
In fanfiction, the relationship might start with curiosity. Kushina could be curious about Naruto's demon side. She starts to study it and in the process, gets closer to Naruto. As she helps him understand and control his powers better, they become friends. Then, as they spend more time together, they realize they have feelings for each other that go beyond friendship. They support each other through difficult times, like when Naruto has to face the negative aspects of his demonic form, and Kushina has problems in the village. This continuous support deepens their relationship.
One plot idea could be that Demon Yusuke has to protect Keiko from other demons who see her as a way to get to Yusuke. Another could be Yusuke trying to balance his demon side with his relationship with Keiko, like when he has to hide his new powers and instincts from her at first. Maybe there could also be a plot where Keiko discovers a way to temporarily become a demon herself to be closer to Yusuke's world.
Yusuke would be extremely angry. He would immediately start looking for Keiko. Yusuke is very protective of her, so he would drop whatever he was doing and go on a full - scale search. He might first try to find Hiei's usual hiding places in the demon world and question any demons he comes across.
Growth is another theme. Yusuke often goes through a lot of changes, and Keiko is there. Fanfictions might show how they both grow as individuals and as a couple in relation to those changes. For example, Yusuke becoming more responsible and Keiko being more understanding.
One way the story could develop is that Yusuke goes into a deep state of grief and denial at first. He might start to isolate himself from his friends and the spirit world as well. He could be haunted by memories of Keiko and constantly wonder if there was something he could have done to prevent her death.