It depends on the specific fanfiction. But it could be the main characters from the Twilight series like Bella, Edward, and Jacob. If it's about a vampire losing control, Edward might be a central character. Since he's a vampire, he constantly has to fight his natural instincts. If it's more about a werewolf - related 'losing control' situation, Jacob would be a likely main character.
Often, in fanfictions related to Twilight, characters like Alice might be involved in a 'losing control' scenario. Alice has the power of seeing the future, and if she were to lose control of that power, it could lead to a very interesting story. Also, Carlisle, as the moral compass of the Cullen family, if he were to lose control in some way, it would be a significant plot point. So he could be one of the main characters in such a fanfiction.
Chapter 500, "Nearing Home Brings More Fear", is the clearest early answer because it turns Jiang Mingyu's situation into a concrete plot event. It opens with The room, white and gleaming, was filled with a faint smell of... and closes on Truth be told, it would be a lie to say he wasn't scared..
Chapter 376, "I'm Just Afraid You Don't Need Me Anymore", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. It opens with The phone call fell into a long silence. and closes on A week passed like this before she finally brought gifts to the....
Chapter 484, "Gu Yu Returns to the Country!", is the clearest early answer because it pushes Jin Siyue into a new phase or setting.
Chapter 503, "How could they let an actor marry in?", is the clearest early answer because it turns Shen Ruobing's situation into a concrete plot event. Early on, The servants standing outside the gilded mahogany gateway were all....
Chapter 569, "Watch How My Sister Torments You!", is the clearest early answer because it turns Guo Xing's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Due to the unequal number of members on Team J and Team Q, the system...
Chapter 509, "Don't Cry Funeral Until Someone Dies", is the clearest early answer because it turns Shang Shuyue's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Because of the commotion from the night before, Jiang Li had...
Chapter 219, "Five Thousand Jin??!!", is the clearest early answer because it turns Wen Jiaojiao's situation into a concrete plot event. The chapter gives readers a real answer by opening on Qi Tianyu was somewhat freaking out.
Chapter 678, "The Biggest Connection in the Circle of Friends!", is the clearest early answer because it turns Shang Shuyue's situation into a concrete plot event. The chapter gives readers a real answer by opening on The landlord stood up, maintaining his smile, "Fifty-eight thousand,...
Chapter 492, "Rewarding You with Some Wild Vegetables", is the clearest early answer because the buildup finally cashes out in a real clash. Early on, Upon hearing the words, Jiang Cheng's face instantly turned pale..
Chapter 749, "What if He Made a Fatal Mistake?", is the clearest early answer because it turns Weibo's situation into a concrete plot event. A useful checkpoint is the way the scene starts with I saw the trending topics." Jiang Li left Weibo and initiated a voice...