Kuina passed away in the manga because of a fatal stumble. This incident left a lasting impression and influenced the plot and characters' developments.
One characteristic is that they often focus on Chishiya's intelligence. Writers like to show how he outsmarts others in various situations. For example, in many fanfics, he's the one who figures out the rules of the deadly games first.
In the Alice in Borderland manga, unfortunately, Chishiya met his end. The circumstances of his death were quite dramatic and tied to the overall plot progression.
The theme of survival in the Borderland also plays a big part. They are both trying to survive in this dangerous world, and their relationship can either be a hindrance or a help in that. In some fanfictions, they have to rely on each other despite their differences to make it through various deadly games.
Well, 'Chishiya and the Lost Memories' is quite popular. In this fanfic, Chishiya wakes up with partial amnesia and has to figure out his past while still being his intelligent self. It has a really engaging plot. Also, 'Chishiya's Allies' is good as it focuses on his relationships with other characters in a more detailed way.
Romance is also a big theme. Writers often explore the relationship between the reader and Chishiya, from the initial attraction to a deeper emotional connection. Some fanfictions focus on how they fall in love in the midst of the chaos of the Borderland.