Dark Hermione's development might involve her exploring the Forbidden Section of the library more and more, getting influenced by the dark magic books. She could start to distance herself from her old friends. Dark Fleur, on the other hand, could be developed by her interactions with other dark wizards. She might be tempted by their offers of power and start to change her values, becoming more ruthless in her pursuit of dark magic.
Often, Dark Hermione's character is developed by showing her descent into the dark side. Maybe she starts to question the limitations of the 'good' magic she has been taught and becomes frustrated. For Dark Fleur, it could be through her desire for more power and recognition. Her beauty and confidence could be turned into tools for her dark pursuits.
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.
Look to Chapter 55 for the clearest answer. It begins with Su Yinwan hadn't expected that, in just one day, Jiang Li's reputation had..., keeps building through Yet she was clearly the wretched cannon fodder of this book., and lands on He Yun's fans were immediately displeased..
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.
If you want the clearest chapter reference, go to Chapter 17. Early on, Jiang Li glanced at Jiang Shixu, who was tense and anxious behind her, and...; later, the chapter leans on Okay, let's go eat." After speaking, she strode forward..
If you want the clearest chapter reference, go to Chapter 11. Early on, After a meeting, Shang Shaojing pinched the bridge of his nose, tired, and...; later, the chapter leans on How is it going with the Jiang Family?" The assistant looked troubled and....
If you want the clearest chapter reference, go to Chapter 23. Early on, Although Jiang Li exerted very little strength, Jiang Yan instantly felt...; later, the chapter leans on The pain twisted his face grotesquely as he dove headfirst into the sofa..
Chapter 25 is the strongest chapter-level answer because it starts from Therefore, from his childhood, Jiang Shixu had felt an extreme aversion to... and follows through on Having worked beside Shen Lan for over a decade, she had been there when....
The best chapter pointer here is Chapter 68, "She finally managed to win a round back!". It opens with Jiang Li carried the noodles straight out of the kitchen and came to the... and closes on But didn't Uncle Zhao say he was still sick?.