One common theme is revenge. After being betrayed, Ash often seeks revenge on those who wronged him, which leads him down the path of evil.
One popular 'ash becomes evil after betrayal fanfiction' is 'Ash's Dark Descent' which explores how Ash's trust is shattered and he turns to the dark side. It has some really intense scenes where you can see his transformation.
A typical sign is his disregard for the well - being of others. Previously, Ash might have been a selfless character, but after betrayal, he may start to use people for his own gain. Also, his appearance might change in the fanfiction. He could start to dress in darker colors or have a more menacing look in his eyes, symbolizing his inner transformation.
Power and growth are also typical themes. Having Reshiram gives Ash new power, and he has to grow in order to handle it properly. The fanfics usually show how he learns to control Reshiram's power and how it affects his battles. For instance, he might face stronger opponents but with Reshiram, he has new strategies to overcome them. Also, the theme of trust is important. After the betrayal, Ash has to learn to trust again, this time with Reshiram, and the story often explores how this trust develops over time.
In some fanfictions, Ash might become evil after betrayal when his trust is shattered. For example, if his friends turn on him for some false accusations, he could start to see the world in a different, more cynical way. This broken trust might lead him to believe that power is the only thing that matters, and he starts to pursue it without regard for the consequences, gradually turning evil.
In 'ash becomes red fanfiction', a common theme might also be discovery. Ash could be on a journey to discover the origin or the full extent of his new red state. This could involve him traveling to new places, meeting new characters, and uncovering secrets about himself or the world he lives in. There could also be themes of acceptance, both from himself and from those around him, as he adapts to this new state.
In 'Ash becomes a Lugia' fanfiction, a recurring theme is the clash between Ash's human nature and his new Pokémon form. His human emotions like determination, loyalty, and compassion are still there but are expressed differently as Lugia. Another theme is the exploration of the Pokémon lore. Since Lugia is a very important and mysterious Pokémon, the fanfiction may dig deeper into the myths and legends surrounding it. Additionally, the theme of growth is present. Ash grows not only in terms of power but also in his understanding of the Pokémon universe as he becomes Lugia.
A common theme is the exploration of power. As Ash becomes Mewtwo, he has to deal with a great amount of power that he's not used to. Another theme is identity. He has to figure out who he is now that he's no longer the regular Ash. Also, friendship is often a theme. His friends may have different reactions to his transformation, and he has to navigate those relationships.
It could be about loss and the aftermath. When ash becomes cold, it might symbolize the end of something, like a relationship or a great adventure. So, themes of grief and moving on might be common.
Another common theme is redemption. Even though he's evil, some fanfictions might explore the possibility of him changing or finding some sort of redemption for his actions. It adds an interesting layer to the character.
Power struggle can also be a theme. If Diantha betrays Ash for power, it shows how power can corrupt and change a person's values. It could also lead to a battle of wills between them, as Ash might try to oppose Diantha's new -found power - hungry ways. This theme often explores the darker side of human (or in this case, trainer) nature when faced with the allure of power.