Most likely Ladybug would be involved. Since Chat Noir has a special relationship with her, any kiss she's involved in would be of great interest to him.
Well, Chat Noir finding out about the kiss in fanfiction could set off a whole chain of events. He might start to question his relationship with Ladybug. If the kiss was with someone he doesn't like, he could become overprotective or try to keep Ladybug away from that person. It could also make him more determined to win Ladybug's heart for himself.
Luka, who has feelings for Marinette (Ladybug), might be heartbroken at first. But he's a kind person, so he might also be a bit relieved in a way, knowing that Marinette has found someone she truly loves. The other kwamis might be all excited, chattering among themselves. Tikki might be a bit worried at first, thinking about how this relationship could change things for Ladybug, but then also be happy for her as she sees how much Ladybug cares for Chat Noir. Plagg, of course, would be making jokes and teasing Chat Noir about it non - stop.
Yes, they do have a kiss scene in some episodes of the cartoon.
Chat Noir is often extremely helpful in fanfics. He uses his superpowers like cataclysm to protect the city and the people he cares about. For example, in many fanfics, he might save a civilian from a falling building by using his agility and acrobatic skills. His helpful nature also extends to his relationships, as he is always there to support Ladybug, whether it's in battles or in dealing with personal issues.
Obviously, Chat Noir himself. And Ladybug, she'd probably be the one to take him to the hospital. His kwami Plagg too, who would be very concerned about his holder. Also, the hospital staff who would be treating him.
A must - read Chat Noir and Ladybug fanfiction is 'Love in the Shadows'. It has a really interesting plot where their love is tested in unexpected ways. 'The Parisian Miracles' is also great as it has a lot of action and at the same time explores the relationship between the two heroes. And 'The Secret of the Miraculous' is a fanfiction that keeps you on the edge of your seat as it reveals some secrets about their powers and how it affects their relationship.
One possibility could be that an akuma's power goes wrong and instead of attacking Chat Noir in a normal way, it turns him into a baby. Ladybug is then left to protect this baby Chat Noir while also trying to find a way to reverse the transformation. She might have to take him to Master Fu for help, all while hiding him from Hawk Moth who could potentially use this situation to his advantage.
He might become more mature as he witnesses the future. Seeing the consequences of actions in the future could make him more cautious and strategic.
One possibility is that his father could be an older, more experienced superhero. Maybe he was a hero in the past and has been in hiding or something, and now he comes back to reunite with Chat Noir.
The author can show it through his actions. For example, he might be less energetic during battles, not making his usual witty remarks. His body language can also change, like slouching or walking slowly. And his interactions with Ladybug could become more withdrawn.