Ash's character development could be a downward spiral at first. He would feel guilty but also try to justify his actions. As the story progresses, he might start to see the error of his ways and try to make up for his betrayal. This could lead to him becoming a more complex and mature character. Cynthia would initially be very hurt and angry. But this could fuel her growth as a character. She might become more focused on preventing future betrayals and become a stronger advocate for loyalty and trust in the Pokémon community.
Ash could develop his ability to understand different Pokemon cultures better. Cynthia, being from a different region, has a lot of knowledge about various Pokemon traditions. Ash could learn from her and in turn, teach her about his own experiences. This exchange could make Ash more worldly and Cynthia more in touch with the broader Pokemon community. In terms of their relationship, they could start to rely on each other more, which would be a big step for both of them character - wise.
Ash could become more mature. In a love - centered fanfiction with Cynthia, he might start to think more about the future and less about just the next Pokémon battle. His relationship with Cynthia could make him more strategic as he learns from her.
You can try fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Ash Cynthia Diantha' on their search bars and you'll likely find a bunch of relevant fanfictions.
Ash could become more patient. Since Togepi is a relatively weak and cute Pokémon at first, Ash has to be patient in training it. This would be a change from his sometimes impulsive nature in battles.
For Ash, he might become more confident in handling extremely powerful Pokémon. Catching Heatran could make him realize his own growth as a trainer. As for Heatran, it could start as a wild and unruly Pokémon but through Ash's influence, it could become more tame and learn to trust humans.
It could be a jealous fellow trainer. There are always those in the Pokémon world who envy the skills and achievements of others. This trainer might see Ash and Cynthia as obstacles to their own glory and decide to betray them to get ahead.
Ash might develop a deeper sense of responsibility. In a fanfiction, if he is in charge of protecting Manaphy, he would have to think more about the consequences of his actions. He could also learn to be more diplomatic, especially when dealing with May's different approach to Pokémon. May could grow in her ability to adapt. For example, if they end up in a situation where there are no contests but only battles, she could learn from Ash how to handle it. Manaphy could develop its psychic abilities further. Maybe it discovers new powers within itself while being with Ash and May, and they help it to control and use these powers for good.
As a Mew fan, Ash might start to study different aspects of Pokémon lore more deeply. This could make him wiser in the ways of Pokémon. He could also develop a more empathetic side towards Mew - like Pokémon, seeing them as not just creatures to battle with but also as beings with their own unique qualities and stories.
Ash could become more understanding of different training methods through Daisy. Maybe he learns from her how to better communicate with his Pokemon on an emotional level.
For Ash, he could develop better communication skills. In the Kalos League, he often gets so focused on battles that he forgets to share his thoughts and feelings with Serena. By learning to communicate better, he can strengthen their relationship. For Serena, she could develop more self - confidence in her own Pokémon training skills. Maybe she starts out doubting herself but as the Kalos League progresses, she becomes more assertive and makes her own mark in the Pokémon training world.