Ladybug would likely feel a sense of guilt too. She might think that she could have prevented this somehow. So, she would not only be focused on stopping Chat Noir's evil actions but also on redeeming him. She would use her intelligence and creativity, which are her strong suits, to come up with a plan to both save Chat Noir and keep Paris safe from any harm he might cause.
She would also be very determined to bring him back to the good side. Ladybug might search for a way to break whatever spell or influence has made Chat Noir evil. She could look for a special power - perhaps from another Miraculous or a magical item - that can reverse the change. And in the meantime, she has to be careful not to get hurt by Chat Noir while still trying to protect Paris.
One possible plot could be that Hawkmoth manages to finally corrupt Cat Noir. He might start causing chaos in Paris, attacking Ladybug instead of helping her. Ladybug would then be in a difficult situation, having to either try to save her partner or defeat him to protect the city.
Well, if Chat Noir dies in a 'Miraculous Ladybug' fanfiction, it could lead to a whole new arc for Ladybug. She might start to question her role more deeply. She could also be filled with guilt, thinking that she should have been able to protect him. And the other characters, like Alya and Nino, would also be affected. They might see Ladybug change in front of their eyes, becoming more withdrawn as she mourns Chat Noir. The balance between the Miraculous powers would be severely disrupted, and this could open the door for new villains or a change in the way the existing ones operate.
It can strengthen their relationship. Ladybug may become more protective of Chat Noir, showing a deeper level of care. For instance, she might be more hesitant to let him engage in battles until he's fully recovered.
It could still be Hawkmoth. Since he's always after the Miraculouses, he might find a way to turn Cat Noir evil and use him as a tool to finally get Ladybug's Miraculous. So Hawkmoth would be pulling the strings in the background while Cat Noir does his dirty work.
In many such fanfictions, Chat Noir might get hurt during a fight with a powerful akuma. Maybe he's outnumbered or faces a new and dangerous power. Writers often use this to create drama and show Ladybug's concern for him.
One possible fanfiction could involve Chat Noir being akumatized after a major argument with Ladybug. Maybe he feels constantly overshadowed or unappreciated. The akuma could enhance his negative feelings, making him more aggressive and less in control. His powers might become distorted, and he could start causing chaos in Paris. Ladybug would then have to find a way to reach out to the real Chat Noir inside the akumatized form and purify the akuma.
Marinette's friends, like Alya, would be heartbroken. Alya, being Ladybug's biggest supporter, would probably feel a sense of loss and confusion. She might start to investigate Ladybug's death, trying to figure out what really happened. The other classmates would also be affected, with the mood at school becoming very somber.
First, create a well - developed OC. Give them a unique personality, backstory, and powers if applicable. For example, an OC could be a shy artist who discovers a new Miraculous. Then, focus on the relationship with Chat Noir. Build it slowly, like having them start as friends and gradually develop deeper feelings. Also, keep the tone and style similar to the original show. Use vivid descriptions of Paris and the action scenes.
One possible plotline could be that he's tricked by a powerful villain. Maybe the villain uses some dark magic to cloud Chat Noir's judgment. For example, they could make him believe that Ladybug has betrayed him, so in his anger and confusion, he turns evil. He might start using his powers for wrongdoings instead of protecting Paris.
Ladybug would be extremely shocked and heartbroken. She has always seen Cat Noir as her partner and friend, and to see him akumatized would be like losing a part of herself. She would be determined to save him at all costs, trying to find a way to communicate with him beneath the akuma's influence.