Since it's about Sonic, Sonic himself is most likely a main character. There might also be his friends like Tails and Knuckles who would be involved in the story.
In a 'dark sonic flash story', Sonic is surely a main character. Given the nature of the story being dark, there could be some new or antagonist characters as well. But Sonic's friends such as Tails, who is his loyal sidekick with his intelligence and mechanical skills, and Knuckles, the strong guardian of the Master Emerald, would probably play important roles too. They would be there to either help Sonic if he's in trouble or try to stop him if he goes too far in his 'dark' state.
The main character is bound to be Sonic in the 'dark sonic flash story'. However, depending on how the story is crafted, other characters could be crucial. Tails, with his ability to build and repair things, would be a great asset in the story. Knuckles, with his strength and connection to the Emeralds, would also be a key figure. And there could be a villain, perhaps an evil wizard or a powerful mechanical being that has the power to corrupt Sonic and set off this 'dark sonic' scenario.
Chapter 702, "This is One Hundred Million, Sign Here!", 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 Bai Lu was at a loss for words. and follows through with Let's see how she handles this now!.
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.
Chapter 840, "How did you meet Miss Jiang?", is the clearest early answer because it turns Rong Junsheng's situation into a concrete plot event. The chapter gives readers a real answer by opening on Jiang Li seemed surprised that she would suddenly speak, paused for a...
Look to Chapter 4 for the clearest answer. It begins with Qiao regretted her words as soon as they left her mouth., keeps building through Whether Jiang Yan can be found or not was another matter., and lands on A little girl acting mysterious, learning fortune-telling, what is this....
Chapter 821, "She Is Qingyang!", is the clearest early answer because it turns Qingyang's situation into a concrete plot event. The chapter gives readers a real answer by opening on The Yu Family Mansion is a square courtyard, second only to the Wang...
Chapter 844, ""Lang", is the clearest early answer because it turns Lang Uncle Zhao's situation into a concrete plot event. It opens with He had been married, but his wife disliked his disability and... and closes on Don't call me randomly!" "Indeed, you really are not my aunt." Gu....
Chapter 21 is the strongest chapter-level answer because it starts from Jiang Li's clear and logical explanation had everyone stunned in place. and follows through on But the implications could be different..
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....
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..
A practical answer is Chapter 23, because it turns the idea into a concrete scene. The chapter opens with Although Jiang Li exerted very little strength, Jiang Yan instantly felt..., adds pressure through So the moment the news broke that Jiang Li was entangling with Shang..., and ends on However.