Hawkmoth is usually the main antagonist. Since he is the one who akumatizes Marinette in the first place. He takes advantage of her negative emotions, like her insecurities or anger, to turn her into an akuma and use her for his own evil plans.
One interesting aspect could be the character transformation. Seeing Marinette, who is usually kind - hearted, turn into an akumatized form offers a fresh take on her personality. It can explore darker emotions and motives that she doesn't typically display.
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.
One possible plot could be Marinette getting akumatized because she feels constantly overshadowed by Ladybug's responsibilities. She might turn into an akuma with powers that are a twisted version of her creativity, like creating chaotic and unmanageable 'designs' that cause havoc in Paris.
There could also be antagonists like Hawk Moth. Since he is the one sending the akumas, he would be involved in the backstory of Marinette's akumatization. He might have a particular reason for targeting Marinette this time, which could be explored in the fanfic. And perhaps some new characters could be introduced as a result of Marinette's akumatized state, like other heroes or villains that are drawn to the chaos she creates.
She might have a dark, twisted version of her usual pink and black color scheme. Maybe her pigtails would be wild and spiky, like they're full of dark energy. Her dress could be torn and tattered, with a more menacing style, and her eyes might glow a deep red color.
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.
Hawkmoth, of course. He's always looking for opportunities to akumatize people, and Ladybug would be a huge prize for him. If he could find the right moment when Ladybug is feeling vulnerable, like after a big defeat or a personal crisis, he could send an akuma her way.
Marinette can react by standing up for herself. She might confront Chloe directly and tell her that her actions are wrong. This shows Marinette's strong character.
The characters' reactions would be quite complex. Adrien would first be in denial, not wanting to believe that Plagg could be akumatized. Then he would feel a mixture of anger and sadness. He might blame himself for not being able to protect Plagg. Marinette as Ladybug would be focused on the bigger picture. She would try to assess the situation quickly and figure out if there are any weaknesses in the akumatized Plagg's powers. Tikki would be worried too, as she has a relationship with Plagg. She might offer some advice to Ladybug based on her knowledge of kwamis.