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.
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.
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.
Look to Chapter 53 for the clearest answer. It begins with Night had fallen over Chixia Village, eerily void of any human sounds., keeps building through Jiang Li followed the village chief's wife down a narrow path leading to..., and lands on Wait, did this kid accidentally fall into the water?].
A practical answer is Chapter 19, because it turns the idea into a concrete scene. The chapter opens with What criminal law is this or that, this was originally none of your..., adds pressure through Brothers, go in and drag this bad woman out!" "I'll see who dares touch..., and ends on Please.
The best chapter pointer here is Chapter 49, "Mysticism Girl Jiang Li". It opens with My phone!!!" Jiang Yan's roar echoed continuously through the mountains in... and closes on Thinking this, Jiang Yan marched forward, pulled Jiang Li down from her....
The best chapter pointer here is Chapter 23, "Became Trending for Being Scolded". It opens with Although Jiang Li exerted very little strength, Jiang Yan instantly felt... and closes on However, just as she buried herself in the study, pouring over mysticism....
The best chapter pointer here is Chapter 16, "He Actually Fell Asleep". It opens with Just as the words fell, Jiang Shixu swiftly cast his eyes downward. and closes on Jiang Shixu panicked, set down the bowl, and reached forward to grab her....
If you want the clearest chapter reference, go to Chapter 75. Early on, The ordinary people in the livestream were all stirred up by this scene.; later, the chapter leans on [Did you see that, did you see that?.
If you want the clearest chapter reference, go to Chapter 21. Early on, Jiang Li's clear and logical explanation had everyone stunned in place.; later, the chapter leans on Jiang Yan had wanted to defend Su Yinwan since his sister's temper in the....
If you want the clearest chapter reference, go to Chapter 36. Early on, But soon, people began to question the authenticity of Jiang Li's bare...; later, the chapter leans on After all, these days, faux bare-faced makeup can be indistinguishable....
If you want the clearest chapter reference, go to Chapter 47. Early on, Qi is actually second???] [Did the production team make a mistake?; later, the chapter leans on Isn't Qi supposed to be first?] [So who's first?.
A practical answer is Chapter 22, because it turns the idea into a concrete scene. The chapter opens with Hearing this, Su Yinwan looked at Shang Shaojing with a face full of..., adds pressure through Why are you yelling so loudly?, and ends on Jiang Li's righteousness temporarily shut Jiang Yan.