Yes, there are some. You can start by looking on fanfiction websites like Archive of Our Own. Just search for 'takashi x rei' and you'll find a variety of stories written by different authors with different interpretations of their relationship.
One great Takashi x Rei fanfic could be 'A New Beginning'. It shows how Takashi and Rei's relationship evolves in the post - apocalyptic world. They face many challenges together and their bond strengthens through shared experiences.
One common theme is the development of their relationship. It could be about how they first meet and start to have feelings for each other. Another theme might be adventure. They could be on a journey together and their relationship deepens during the adventure.
Survival scenarios are also quite common if these characters are from a post - apocalyptic or action - oriented anime. They might have to rely on each other in dangerous situations, which can bring out different aspects of their relationships. For example, Takashi might have to protect Rei and Saeko from threats, and this can strengthen or test their bond.
I'm not sure which specific rei x takashi fanfiction is the best. But often, on platforms like FanFiction.net, you can find different genres. It could be a love - story type of fanfiction where their relationship develops in a sweet and heartwarming way. You should look for stories with high ratings and positive reviews.
To write their character development well, first understand their starting points. Rei starts off as a strong - willed and somewhat cold girl, while Takashi is more of a regular guy thrust into a leadership role. As the story progresses, for Rei, show how her experiences soften her edges. For example, when she and Takashi save each other from zombies, she could start to open up more emotionally. For Takashi, show his growth in confidence as a leader. Maybe he starts making bolder decisions when it comes to protecting Rei and the rest of the group.