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Yusuke would likely become more withdrawn.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.
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.
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.
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.
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.
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.
One possible character could be Keiko's father. He is a powerful and influential man in their community. When he finds out about Keiko's kidnapping, he uses all of his resources, including his connections with the local police and his own private security force, to try and rescue her. Then there could be a detective, Inspector Kenji, who is assigned to the case. He has a reputation for being tenacious and smart, and he starts to piece together the clues about Rando's identity and motives.
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.
There's this fanfic called 'Together Always' which is really interesting. It delves into how Yusuke and Keiko handle the challenges in their relationship. It shows Yusuke's growth as a character in the context of his love for Keiko. Keiko, on the other hand, is depicted as being very supportive of Yusuke's journey.