Fem Daisuke could betray her family in the fanfiction. It might be that she gets involved with a group or ideology that goes against her family's values. This would cause a rift not only between her and her family members but also within the community they are part of. For instance, if her family is known for their loyalty to a certain cause and she betrays that by siding with the opposition, it would be a major source of conflict.
She could betray her mentor. In many fanfiction stories, a mentor - student relationship is crucial. If Fem Daisuke betrays her mentor, it could be a huge turning point. Maybe she steals an important item or knowledge from her mentor that was meant for a greater good and uses it for her own selfish purposes.
Well, it could be about Fem Daisuke breaking a promise to a close friend in the fanfiction world. Maybe she promised to keep a secret but then revealed it, causing a lot of trouble and hurt feelings among the characters.
Draco Malfoy is a likely candidate. He's often portrayed as a rival to Harry in the original series, and in fem Harry fanfictions, he might be even more likely to betray her. He could see Fem Harry as an easy target to gain more power or status within the Slytherin house. Another one could be Pansy Parkinson. She's known for her mean - spiritedness, and she might betray Fem Harry out of envy or just to cause trouble.
One likely character could be a rival trainer. They might see Fem Ash as a threat and betray her to get an edge in competitions. Another could be a so - called friend who is jealous of Fem Ash's success.
One potential antagonist could be a new and mysterious organization that is similar to Akatsuki but with different motives. They might be interested in using Daisuke to open a portal to a different dimension or something like that. They would be constantly trying to kidnap Daisuke and force him to cooperate. Also, Orochimaru or his remaining followers could be antagonists. Even though Orochimaru has changed somewhat in the series, there could still be some of his followers who are more radical and want to use Daisuke for their dark experiments, believing that he has some special chakra or ability that could help them achieve their twisted goals.
One character who could betray Davis is Ken. Maybe Ken feels that Davis is too impulsive and is leading the group in the wrong direction. So, in an attempt to take over and lead in what he believes is a better way, he betrays Davis.
A dark magic - using villain from the MLP universe could also be the one to betray Twilight at the wedding. This villain might have infiltrated the wedding planning under a false guise. They could then use their magic to turn Twilight's friends against her during the wedding, making it seem like Twilight has done something wrong when in fact she is the victim of betrayal.
His rivals could be the ones to betray him. For example, Gary Oak might see Ash's godlike powers as a threat to his own status and decide to plot against him. Gary could spread false rumors about Ash to turn others against him.
Jealousy could be a motive. If Fem Sasuke sees Naruto getting all the attention and praise while she feels overlooked, she might betray him out of spite.
Fem Daisuke might choose to simply ignore the insults. Fanfiction is a very personal creative output, and she doesn't need to let negative comments from others get to her. She can focus on her own enjoyment of writing it and sharing it with those who appreciate it.
It could be Gray. Although they are friends most of the time, in a fanfiction, there could be a plot twist where Gray's envy of Natsu's abilities gets the better of him, and he betrays Natsu. Maybe Gray thinks that by getting rid of Natsu, he can be the top mage in their guild.