One common theme is the contrast between Master Frown's negative nature and Unikitty's positive one. Writers might explore how they interact in different situations, like Master Frown trying to cause trouble while Unikitty tries to stop him and turn the situation around.
A possible plot could be that Master Frown, who is usually a villain, has a change of heart when he meets Unikitty. Maybe he gets lost in Unikingdom and Unikitty helps him. As they spend time together, he starts to see the world in a different way and they develop an unlikely friendship.
Some dedicated fan communities on social media platforms might also be a good place to look for Master Frown and Unikitty fanfiction. For example, there could be groups on Facebook or Reddit where fans share their works or recommend good fanfictions they've read.
Their relationship can be developed through shared experiences. For example, if they get stuck in a magical maze together, they have to rely on each other to get out. This builds trust between them bit by bit.
Well, a plot could involve Unikitty getting lost in Master Frown's territory. Master Frown at first wants to use this to his advantage, like getting ransom from her friends. But as time passes, he starts to feel a strange connection with her. He starts to protect her from other threats in his world, and in the process, they form an unlikely friendship that challenges his whole view of being evil.
You can try fanfiction websites like Archive of Our Own (AO3). There are often many fan-created works there. Just search for 'Master Frown x Unikitty' in the search bar, and you'll likely find a variety of stories.
Of course. Japanese light novels are written in Japanese and thus are composed mainly of Japanese words. They might incorporate a few foreign words or modern slang, but the core language is Japanese.