The main characters could also be secondary characters from both universes that are given a more prominent role in this fanfic. For example, Shikamaru from Naruto could be a key player. He's known for his intelligence, and in this fanfic, he might be the one to figure out the true nature of the 'twilight key'. From Twilight, Jacob could be involved. He's strong and has his own set of powers. The two of them might be at odds or working together in relation to the key and the events it sets in motion.
I don't know for sure as it's a fanfic. It could have Naruto and Sasuke from Naruto, and perhaps Edward or Bella from Twilight if it's following a more traditional combination of characters from both series.
Since it's a fan - made work, the main characters could be entirely new characters created by the author. However, if it adheres to the general idea of combining Naruto and Twilight, we might expect to see Naruto as a central figure. He could be on a quest related to the 'twilight key'. And maybe he teams up with a Twilight character like Alice, who has the ability to see the future. They could be the main characters trying to figure out the mystery of the key and how it affects both their worlds.
Chapter 350, "Methods to Improve the Peak of Cold Resistance, Moao's 10th Town, Beishuo Yang Fa, Nightmare, Lu Yun of East Ridge", is the clearest early answer because it turns Nightmare Monster's situation into a concrete plot event. It opens with Hmph, the Nine Towns have had an agreement long ago: if a Xianyang... and closes on Evidently, the other party didn't believe his self-introduction just....
Chapter 341, "Moao's Nine Garrisons, the Inspection System and Zhenyu, Li Hu and Hong Yuru (Part 2)", is the clearest early answer because it turns Cold Resistance Level's situation into a concrete plot event. The setup is Lord Wang is not wrong, but not entirely right either., but the pressure keeps building through The Nine Towns all adopted the inspection system first created by....
Chapter 219, "Five Thousand Jin??!!", is the clearest early answer because it turns Director Sun's situation into a concrete plot event. The chapter turns that into something concrete by moving from Qi Tianyu was somewhat freaking out.
Chapter 137, "Lai Fu's Death Has Another Cause", is the clearest early answer because the violence around Yintang becomes final instead of hypothetical. The chapter turns that into something concrete by moving from Jiang Li personally went to Lai Fu's tomb and performed a small...
Chapter 350, "Methods to Enhance the Peak of Cold Resistance, Moao's 10th Town, Beishuo Yang Fa, Nightmare, East Ridge Lu Yun", is the clearest early answer because it turns Great Xia's situation into a concrete plot event. The chapter gives readers a real answer by opening on He didn't care whether Zhu Yang on the ground understood or not. and ending on The construction cost of an eight-meter wide straight road is not....
Chapter 226, "Live Fortune Telling!", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. The setup is What was all that before then?, but the pressure keeps building through Jiang Li quickly tapped the screen twice, and a red packet link that....
Chapter 245, "More Than Meets Your Match!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Jiang Yan became even more smug upon hearing this.
Chapter 16, "He Actually Fell Asleep", is the clearest early answer because it turns Jiang Shixu's situation into a concrete plot event. It opens with Just as the words fell, Jiang Shixu swiftly cast his eyes downward.
Chapter 178, "The Group's Favorite Belongs to No One but Her!", is the clearest early answer because it turns Wen Jiaojiao's situation into a concrete plot event. Early on, Wen Jiaojiao is just too unreasonable, why does she have to single....
Chapter 156, "Group Morning Run!", is the clearest early answer because it turns Shu Yan's situation into a concrete plot event. Early on, Flash Change, who was always tardy, was late again today.. By the end, Soon after, the meditative He Yun also walked over..