Voldemort is also a key character. If Harry is switching sides, Voldemort would be the one he's switching to, at least initially. Voldemort would likely try to manipulate Harry further, using his newfound ally to gain more power. Dumbledore, even if he's dead in the original series, can still have a major influence in the fanfiction. His past actions and words might be re - examined by Harry as he makes his decision to switch sides, and his spirit or memories could still guide Harry in some way.
Obviously, Harry Potter himself is the central character as the story revolves around his switching sides. Hermione and Ron also play significant roles. When Harry starts to show signs of siding with the dark side, Hermione might try to use her intelligence to figure out what's wrong and bring him back. Ron, on the other hand, could be more impulsive in his reactions, either trying to fight Harry if he becomes a threat or trying to persuade him to return to the good side.
Snape is another character that could be deeply involved. Given his complex relationship with Harry in the original story, in this type of fanfiction, Snape could either be a force trying to stop Harry from going too far to the dark side, or he could be misinterpreted by Harry, which could contribute to Harry's decision to switch sides in the first place.
A practical answer is Chapter 8, because it turns the idea into a concrete scene. The chapter opens with The resentment that had barely calmed among the crowd surged back two..., adds pressure through Why keep harping on the fact that he wasn't a decent man?, and ends on Other people with a.
The best chapter pointer here is Chapter 3, "Madam, the eldest young master has been missing for over half a month". It opens with Qiao was still annoyed at the attitude of this unknown girl when she heard... and closes on Jiang Yan, forced into a marriage with someone he did not love, had....
Chapter 674, "Staring at Shu Yan", is the clearest early answer because it turns Lin Manru's situation into a concrete plot event.
A practical answer is Chapter 13, because it turns the idea into a concrete scene. The chapter opens with Sensing that something was off with Jiang Yan, Jiang Yan dismissed the..., adds pressure through He probably had a good guess who it was., and ends on How could this happen..." Lin Manru.
Look to Chapter 39 for the clearest answer. It begins with Qi Tianyu had been looking forward to watching Jiang Li's drama unfold., keeps building through But to his surprise, Wen Jiaojiao, who seemed to possess some strength,..., and lands on His original intention was just to make Jiang Li back.
Chapter 866, "Why don't you die in his place?!", is the clearest early answer because it turns Didn's situation into a concrete plot event. The chapter gives readers a real answer by opening on In Shuiyun Temple, Changqing silently watched everything happening in...
Chapter 791, "There's Actually a Suona?!", is the clearest early answer because it turns Chen Taifan's situation into a concrete plot event. The chapter turns that into something concrete by moving from The lights in the audience suddenly went out, and immediately, the...
Chapter 835, "Is this your way of handling things? So pathetic.", is the clearest early answer because it turns Mommy's situation into a concrete plot event.
Chapter 836, "It's been a long time since I last saw Jin Siyue", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. The chapter gives readers a real answer by opening on You!" Rong Junsheng seemed to have heard the commotion over here and...
If you want the clearest chapter reference, go to Chapter 2. Early on, On the top floor of the Financial Building, a gathering of dark clouds...; later, the chapter leans on How dare Jiang Li hang up his phone?.