Yusuke would likely be shocked at first that Hiei, who he considered a friend, would do such a thing. But then his determination to save Keiko would kick in. He'd probably call on his other friends, like Kuwabara, to help him. He would also use his detective skills, if any, to figure out where Hiei might have taken Keiko. He'd be relentless in his pursuit, not stopping until he found her safe and sound.
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.
In one scenario, Hiei might kidnap Keiko to use her as bait to draw out Yusuke. He could be forced by some powerful enemy who wants to get revenge on Yusuke. Hiei, being a rather complex character, might not be entirely happy about this but feels he has no choice. He would keep Keiko in a hidden location, perhaps in the demon world where it's hard for Yusuke to reach easily.
Yusuke would likely become more withdrawn.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.
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.
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.
One possible plot could be that Keiko is caught in the crossfire of a demon battle that Yusuke is involved in. Yusuke, being a spirit detective, has many enemies. Maybe a powerful demon targets Keiko to get back at Yusuke. After her death, Yusuke could go into a deep state of despair and vow revenge. He might journey to the spirit world to find a way to bring her back, facing numerous challenges along the way.
If Karasa kidnaps Keiko in a fanfiction, the story could go in many directions. For instance, Keiko might turn out to be much stronger than Karasa expected. She could use her wits and any special abilities she has to try and escape. Or perhaps there are other characters who will come to Keiko's rescue. They could be friends, family, or even strangers who happen to be in the right place at the right time.
Yes, there are quite a few. You can start by looking on fanfiction platforms like Archive of Our Own. Just search for 'yusuke and keiko love' and you'll find a variety of stories written by different authors with different takes on their relationship.
To write this fanfic, you need to understand the characters well. Yusuke is tough on the outside but has a soft heart. Keiko is strong - willed and intelligent. Use these traits in the story. For instance, when they face difficulties during the pregnancy, Yusuke might want to solve them with his fists at first, but Keiko would remind him to think rationally. Also, add some cute moments, like Yusuke feeling the baby kick for the first time.
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.