The main characters would surely include Ash. In Pokemon Red, the rival character, Gary (or whatever name you choose for the rival in the game), could also play a major role. And of course, the various gym leaders from Pokemon Red. They could have different encounters with Ash, some might be friendly and help him, while others could be more challenging opponents. Also, Professor Oak might be involved, as he is connected to both Ash's world and the world of Pokemon Red in some ways.
Ash is a given as a main character in the 'pokemon red meets ash fanfiction'. Then, in the Pokemon Red side, there could be the Pokemon themselves as important characters. For instance, the Legendary Pokemon like Mewtwo which has a big story in Pokemon Red. The shopkeepers or other NPCs from Pokemon Red could also be part of the story, adding to the unique flavor of the fanfiction. They could provide Ash with items or information that are crucial for his adventure in this combined world.
Chapter 119, "Running Out of Materials, Archery Contest [Warm Sun Giant's Additional Chapter 2/10]", is the clearest early answer because the buildup finally cashes out in a real clash. The chapter gives readers a real answer by opening on If one can win first place in the competition, it would be a great... and ending on Smiling, Xia Hong waved his hand, originally not wanting to try, but....
Chapter 122, "Finally Started to Withdraw [Three in One]", is the clearest early answer because it turns Cold Beasts's situation into a concrete plot event. Early on, Everyone, find your targets, quickly!" Though the white fog was.... By the end, Especially from having dealt with the rodents a few times, their....
Chapter 192, "Beehive Predicament, Surge in Earth-digging Realm", is the clearest early answer because Wu Tianxing finally turns buildup into a real advancement. The chapter turns that into something concrete by moving from Wu Tianxing's face turned grave upon hearing this, and he said... to Wu Tianxing remained silent, his gaze drifting elsewhere..
Chapter 117, "Teng Jiao Saliva and Poison Source [Combined Chapters]", is the clearest early answer because it turns Cold Beast's situation into a concrete plot event. Early on, Although Xia Hong understood this principle, he had no way to improve.... By the end, Inside that meat, there was a pure white round pearl, emitting a....
Chapter 180, "Exchange Meeting, Murder, Great Xia Gospel", is the clearest early answer because it turns Chen Hua's situation into a concrete plot event. Early on, Some things can cause a chain reaction.. By the end, The monthly exchange meetings have made the leaders of the eleven....
Chapter 119, "Running Out of Materials, Archery Contest [Warm Sun Giant's Additional Chapter 2/10]", is the clearest early answer because the buildup finally cashes out in a real clash. The chapter turns that into something concrete by moving from The process isn't complicated, but it requires careful attention. to This sense of achievement isn't something that can be easily....
Chapter 122, "Finally Started to Withdraw【Three in One】", is the clearest early answer because it turns Iron Arrows's situation into a concrete plot event. The chapter turns that into something concrete by moving from The broadsword can be used, but against a swarm of rodents, its... to The lead two rodents were nailed by the Iron Arrows, the poison sacs....
Chapter 99, "Life is Improving", is the clearest early answer because it turns Xia Hong's situation into a concrete plot event. The setup is Zou Yuankai's face wound was from last night; it was simply wrapped..., but the pressure keeps building through It wasn't even close to scabbing, and the flesh was not yet healed..
Chapter 182, "Red Wood, Eye of Truth, Three Joys Arrive at the Door", is the clearest early answer because the hidden truth stops feeling like mere subtext. It opens with Everyone turned their heads to look behind them, bowing respectfully... and closes on The people of Great Xia looked at Xia Chuan, who appeared....
Chapter 104, "The Camp's Strength, Rapid Enhancement (2-in-1 Mega Chapter)", is the clearest early answer because it turns Great Xia's situation into a concrete plot event. The chapter turns that into something concrete by moving from As Xia Hong had predicted, ten days after Luo Yuan's breakthrough,... to But the increasingly busy scene in the valley camp made Xia Hong feel....