I think it's because of the contrast between their personalities. Yusuke is this wild, unruly character and Keiko is more reserved and kind. Their relationship has a lot of potential for interesting stories.
Yusuke would likely become more withdrawn.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.
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.
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 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.
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.
There could also be a fanfiction where Yusuke and Keiko get trapped in a parallel universe. In this universe, things are slightly different but their feelings for each other remain the same. They have to adapt to the new environment while also trying to find a way back home. Their love and determination keep them going through all the difficulties they encounter in this strange new world.
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.
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.
Another theme is sacrifice. Yusuke has sacrificed a lot in his life, and Keiko is often by his side. Fanfics may show how they both make sacrifices for each other and for their relationship. For example, Yusuke might sacrifice his time or safety to protect Keiko, and she might sacrifice her own desires for Yusuke's well - being.
There's also 'The Unspoken Bond'. This fanfiction delves into the unsaid feelings between Yusuke and Keiko. It has a really nice build - up of their emotional connection through various events.