Well, Ash is a given. But then there are other characters like May. May was another important female character in Ash's journey. If the fanfiction brings her back into the picture, Serena might be jealous of the memories Ash has with May. There could also be male characters involved in a way that makes Serena jealous. For example, if there's a male trainer who is really good friends with Ash and they go on adventures together without Serena, she could be jealous of their friendship and the time Ash spends with him instead of her.
Definitely Ash, as he's the center of Serena's affections. But also, there could be new female characters created specifically for the fanfiction. These new characters might be trainers who are really good at Pokemon battles and catch Ash's attention. Serena would then become jealous of their skills and the attention Ash gives them. Additionally, any of Ash's Pokemon that seem to have a special connection with another character could also be part of the scenario that makes Serena jealous. For example, if Pikachu seems to like a new girl more than Serena, that could be a reason for her jealousy.
A practical answer is Chapter 227, because it turns the idea into a concrete scene. The chapter opens with Your Four Pillars of Destiny is very strong, with wealth, authority, and..., adds pressure through As long as you're in that house, they will never be at ease;
A practical answer is Chapter 234, because it turns the idea into a concrete scene. The chapter opens with Given the vast mystical powers Jiang Li had displayed before, he was..., adds pressure through I didn't see any trace of wrongdoing in his birthdate, you can all relax., and ends on His.
The best chapter pointer here is Chapter 215, "Overthinking is a Disease, It Needs Treatment!". It opens with Jiang Li let out a light sigh and tried to adjust her posture. and closes on He originally just wanted to see if there was a chance to complete his....
A practical answer is Chapter 226, because it turns the idea into a concrete scene. The chapter opens with What was all that before then?, adds pressure through The moment Jiang Li saw it, she confirmed her guess even more., and ends on Are you sure you can be on the list of winners?] Zhou Xu: [?.
Chapter 251 is the strongest chapter-level answer because it starts from If Du Qinglian was somewhat angry just a moment ago, now she was simply at... and follows through on The moment the connection was successful, before even appearing on camera,....
Chapter 230 is the strongest chapter-level answer because it starts from Shang Shaojing's expression wasn't looking too good either. and follows through on If he knew that your stepmother was secretly prompting you towards a.... The payoff is that the chapter closes with "Liu Linmei!
If you want the clearest chapter reference, go to Chapter 197. Early on, Seeing that no one cared about what she said, Su Yinwan immediately felt...; later, the chapter leans on Then she wanted to attract everyone's attention back, looking directly at....
If you want the clearest chapter reference, go to Chapter 252. Early on, In fact, Jiang Yan's shirtless appearance this time was entirely Jiang...; later, the chapter leans on Her exact words were, "Since you dislike wearing clothes so much, you....
Chapter 185 is the strongest chapter-level answer because it starts from He was so anxious that he immediately picked up the megaphone. and follows through on The next, she'd complain that the ground was too slippery to walk on..
If you want the clearest chapter reference, go to Chapter 235. Early on, Chasing Wind Youth was clearly becoming anxious, and as a result, his...; later, the chapter leans on I, I didn't, I'm not, I, I haven't been involved in illegal activities in....