No, it is not appropriate. 'Miraculous Ladybug' is a popular animated series aimed at a wide audience including children. Writing sexual fanfic about the characters goes against the family - friendly nature of the show and can be considered inappropriate and disrespectful to the original work and its fans.
Marinette and Cat Noir have a complex and endearing love story. Marinette is in love with Adrien, not realizing that Adrien is Cat Noir. Meanwhile, Cat Noir is drawn to Ladybug, who is actually Marinette in disguise. They fight side by side against villains in Paris. Their relationship is filled with misunderstandings, yet there are also tender moments when they see the good in each other. It's a love story that is slowly evolving as they get to know each other better in their superhero and civilian identities.
One interesting Marinette and Cat Noir fanfiction could be where they have to face a new villain that has powers they've never encountered before. In this story, Marinette might struggle with her identity as Ladybug while also dealing with her feelings for Cat Noir. Another could be a time - travel fanfic, where they go back in time and meet their predecessors as superheroes.
One key moment is when they first fight together as Ladybug and Cat Noir. It's the start of their relationship as superhero partners. Another is when Cat Noir saves Marinette in her civilian form, which shows his care for her without knowing she's Ladybug. Also, the times when Marinette as Ladybug has to rely on Cat Noir during tough battles are important. These moments build the foundation of their love story.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of Marinette and Cat Noir fanfictions, with different tags to help you find exactly what you're looking for, whether it's a specific genre like romance or action, or a particular plot type. Another option is FanFiction.net, which also has a decent number of stories in this fandom.
There's also 'A Kiss in the Rain' which is quite well - loved. In this story, the setting of the rain adds a lot of romantic tension. It shows how their relationship develops to the point of that kiss in a very natural way. It also has some really sweet dialogue between Cat Noir and Marinette that makes the kiss even more meaningful. The story goes into their insecurities and how they overcome them to reach that moment of closeness.
Write about their friendship development. Maybe explore how they gradually become better friends outside of their superhero duties, like having normal conversations or sharing hobbies. It can add depth to their relationship in a positive and appropriate way.
Some Miraculous Ladybug fan communities on Reddit could also be a good source. Subreddits dedicated to the show might have threads where fans discuss and share their favorite fanfictions that involve Cat Noir and Marinette kissing. You can ask in those communities as well if you don't immediately find what you're looking for.
Another reaction could be that Marinette pretends to cooperate with Adrien at first. By doing this, she can buy time and look for an opportunity to escape or get help. She might act as if she is okay with what is happening while secretly planning her next move, like leaving a clue for someone to find her or finding a way to disable Adrien temporarily.