A believable main character should have flaws. Maybe they are overly confident because of their reincarnation knowledge, but this often gets them into trouble. Their relationships with other characters should also be complex. They could be friends with some devils, but also have rivalries. And their growth throughout the story is important. They should learn from their mistakes and become a better version of themselves as the fanfiction progresses.
To create a believable main character in Highschool DxD reincarnation fanfiction, start with giving them a clear motivation. For example, if they were bullied in their previous life and now have power, they want to stop bullying in the devil world. Their reactions to new situations should be consistent with their past experiences and personality.
The character's backstory is crucial. If they are reincarnated, explain how their previous life memories affect them. For instance, if they were a scientist in their past life, they might approach magic in the Highschool DxD world from a scientific perspective. Also, their dialogue should match their character. A reincarnated character might use modern slang which stands out in the more formal or archaic language of the Highschool DxD setting, but not too much that it becomes unbelievable.
Chapter 469, "He Must Not Tolerate This Calamity", is the clearest early answer because it turns Children Palace's situation into a concrete plot event. A useful checkpoint is the way the scene starts with There was no other choice, so Jiang Li and the others hastily turned...
Chapter 313, "Grandpa, You Scared My Guest", is the clearest early answer because it turns Jiang Ye's situation into a concrete plot event. The chapter gives readers a real answer by opening on When he was brought in, Zhou Wenshu was still in a fog.
Chapter 469, "He Must Not Tolerate This Calamity", is the clearest early answer because it turns Yu Li's situation into a concrete plot event. The chapter turns that into something concrete by moving from There was no other choice, so Jiang Li and the others hastily turned...
Chapter 548, "A Guy Named Gu Yu", is the clearest early answer because it turns Chengcheng's situation into a concrete plot event. A useful checkpoint is the way the scene starts with No, we can't," Su Yinwan blurted out subconsciously.
Chapter 733, "Those People from the Jiang Family Are More Stupid than the Next!", is the clearest early answer because it turns More Stupid's situation into a concrete plot event. The setup is Okay, Sister, you should go to sleep now., but the pressure keeps building through Don't stay up late;
Chapter 286, "I Won't Let Anything Happen to Either of Us", is the clearest early answer because it turns Primordial Forest's situation into a concrete plot event. It opens with Throughout these reincarnations, Zhao Lang had gradually discerned a...
Chapter 475, "Doting on the Man You Love to the Fullest", is the clearest early answer because it turns Hua University's situation into a concrete plot event. The chapter turns that into something concrete by moving from On the other side, Jiang Yan and Shang Shuyue also set off.
Chapter 643, "Jiang Li Dominates the Trending Searches!", is the clearest early answer because it turns All's situation into a concrete plot event. It opens with Qi Tianyu joyously carried his construction to the side to wait for...
Chapter 434, "The Identity of the Feng Shui Master Can't Be Hidden Anymore", is the clearest early answer because the hidden truth stops feeling like mere subtext. Early on, He thought Jiang Li was just talking, but to his surprise, there....
Chapter 659, "Bodyguard Agreement", is the clearest early answer because a shaky connection becomes usable in the plot. A useful checkpoint is the way the scene starts with Besides, how easy is it to get into the Gu Family? and follows through with He only hated himself, his own incapacity..