Yes, there are likely some fanfictions out there that might include a kiss between Kai and Aichi. You can try searching on popular fanfiction websites like Archive of Our Own or FanFiction.net.
One way is to show them outside of cardfights. Maybe they start hanging out more, like going to card shops together to look for new cards. They could talk about their lives, their dreams related to cardfighting. Also, if one of them gets into trouble, like Aichi being bullied by some other cardfighters, Kai could come to his rescue. This would strengthen their bond. Another aspect could be that they start training each other's decks, sharing their insights on different card combinations. Through these interactions, their relationship can grow in a more personal and friendly way.
As I said before, Archive of Our Own and FanFiction.net are good places to start. Just use their search functions and type in relevant keywords like 'Kai and Aichi' and 'kiss' to see if there are any stories that match.
There's a fanfic called 'Kai and Aichi: A New Adventure'. In this one, they embark on a journey to discover a new type of card that could change the world of Cardfight Vanguard. The story is filled with action, friendship, and a bit of mystery as they face various opponents and obstacles along the way.
One popular kai x aichi fanfiction might be those that explore their friendship evolving into a deeper emotional bond. For example, stories where they support each other through difficult cardfights and gradually realize their feelings for one another.
Another option is to check out some Cardfight!! Vanguard fan communities on social media platforms like Reddit or Tumblr. Sometimes fans will share their own fanfictions or recommend good ones they've read. You can also post a request for kai x aichi fanfiction in these communities and see if anyone can point you in the right direction.
One idea could be a story where Kai and Aichi go on a journey through a magical lemon grove. In this grove, they discover that the lemons have special powers that can heal the broken relationships in their world. Another could be a modern - day AU where they run a lemonade stand together and through that, they learn more about each other's dreams and fears.
Well, one possible plot could be that Tasuku and Kai initially have some sort of rivalry when they meet. Maybe they're competing in a card game. Aichi, being the kind - hearted one, tries to ease the tension between them. As the story progresses, they find out they have more in common than they thought and end up teaming up against a common enemy in the card game world.
It could be Ren. Ren is often portrayed as a bit of a troublemaker in the card - playing universe. He might see Tasuku, Kai and Aichi as a threat to his own status as a top player and try to undermine them at every turn.
They could be friends having an intense argument in a story. For example, they might be arguing about a card game strategy, where their strong opinions clash but in the end they learn from each other.