Often in these fanfics, a plot twist could involve Issei being influenced by an ancient and evil artifact. This artifact might cloud his judgment and make him see his friends as enemies. Or perhaps Issei has a hidden past that resurfaces, and he believes that by betraying his current friends, he can atone for something in his past. This could lead to a complex web of relationships and events within the fanfic.
One common plot twist could be that Issei betrays his friends for power. Maybe he's lured by a powerful devil who offers him unimaginable strength on the condition that he turns against his comrades. Another could be a misinformation scenario where Issei is tricked into believing his friends have betrayed him first, so he retaliates.
One common plot twist could be Issei's sudden loss of control over his Dragon God powers. This can create a lot of chaos and internal conflict for him as he tries to regain control and not harm those around him.
A common plot element is the shock of betrayal. Issei is usually shown as a trusting character, so when he is betrayed, it's a huge emotional blow. Another element is his struggle to come to terms with it. He might question his own judgment and relationships. Also, often there is a part where he decides whether to seek revenge or forgiveness. For example, in some fanfictions, he initially wants revenge but then realizes that forgiveness might be a more noble path.
One common plot twist could be Batman being betrayed by a close ally like Robin. Maybe Robin gets brainwashed by a powerful villain and turns against Batman. Another could be Batman's trusted butler Alfred being forced to betray him under some extreme threat. For example, a villain might hold Alfred's family hostage and make him leak Batman's secrets.
One common plot could be Issei being betrayed by his friends in the Occult Research Club. Maybe they believe false rumors about him being a traitor to their cause. For example, an enemy might plant evidence that makes it seem like Issei is secretly working for a rival faction. Another plot could involve Issei being betrayed by a love interest. She might be forced to turn against him due to some dark magic or family pressure.
One common plot twist could be that a close friend or ally, who Ichigo thought he could trust completely, turns out to be working for the enemy all along. For example, Orihime might be brainwashed by a powerful Hollow to feed information about Ichigo's whereabouts and plans to the Hollows. Another could be that the Soul Society, which Ichigo has often helped, decides to turn on him due to some false accusations of him being too dangerous. Maybe they believe he is on the verge of losing control of his Hollow powers and decide to eliminate him before he becomes a threat to both the living world and the Soul Society.
Another possibility is that Edward betrays Bella by revealing her secrets to their enemies. Maybe he was forced to do so under some sort of blackmail from a powerful and malicious vampire or other supernatural being. In this case, Bella would have to face the consequences of having her secrets exposed while also dealing with Edward's betrayal.
One common plot twist could be that the person she trusts most, like her best friend, betrays her for power. Maybe her friend is offered a high - ranking position in a magical guild or something and has to sacrifice Lucy to get it. Another could be that a love interest betrays her. He might be secretly working for an enemy and using Lucy to get information about her family or her magic.
Often, a power - hungry character might betray Issei. In the world of the fanfiction, power is crucial. So, someone who wants more power than Issei may see him as an obstacle. They could betray him by stealing his power - up opportunities or even directly stealing his power if there's some sort of magic or ability transfer in the story. Also, a character with a hidden agenda related to the overarching plot of the fanfiction world, like a secret organization trying to control the balance of power between devils, angels and fallen angels, could use Issei and then betray him when he has served their purpose.
Another plot twist could be that Percy discovers he has some hidden power or heritage as a result of the betrayal. For instance, he might find out that he has a connection to an ancient and powerful force that the gods were afraid of, which is why they betrayed him. This new power then becomes a key element in his journey to either seek revenge or prove his worth.
First, you need to establish a good backstory for Issei. Understand his character traits, his relationships, and his goals. Then, come up with a strong reason for his betrayal. It could be something like a broken promise or a misunderstanding. Next, plan out how his betrayal will affect the other characters. Will they be shocked? Angry? Sad? Write their reactions realistically.