Her romantic partner is a likely betrayer. In many stories, love can turn sour. He might betray Mikan for another person who he thinks has more to offer, like more wealth or a higher social status. Or he could betray her because he's involved in some shady business and doesn't want Mikan to find out, so he does something to push her away.
It could be her best friend. You know, sometimes jealousy can rear its ugly head. If Mikan has something that the friend desires, like popularity or a special talent, the friend might betray her out of envy. For example, the friend could start bad - mouthing Mikan behind her back to gain more attention for themselves.
It could be an enemy who has infiltrated his team disguised as a friend. For example, in a crossover with Fullmetal Alchemist, an alchemist with the ability to change appearance might pretend to be on Kakashi's side but then betray him.
It could be his friend, like Mitsuki. Mitsuki has his own secrets and goals, and in some fanfiction, he might be forced to betray Boruto for the sake of some greater good in his mind. For instance, if he has to choose between following Orochimaru's orders and staying loyal to Boruto.
In some versions, it could be an evil version of one of Tails' friends. Maybe through some dark magic or experiment gone wrong, Sonic or another friend turns against Tails. Their altered state makes them see Tails as an enemy, and they start to undermine him and try to harm him, which would be a huge betrayal for Tails.
Well, in a 'mikan sakura betrayed fanfic', she could be betrayed by her family. Say her sister steals an important opportunity from her, like a chance to study abroad or participate in a big competition. It would be a huge betrayal as family is supposed to support each other. Also, in a magical or fantasy version, Mikan could be betrayed by her mentor in the art of magic. The mentor might use Mikan's trust to steal a powerful magical artifact that Mikan was supposed to protect.
It could be a jealous demigod. Maybe someone who is envious of Percy's popularity and Nico's unique powers. This person might see an opportunity to get rid of them and take their place in the demigod hierarchy.
It could be Valerie. She has had a somewhat rocky relationship with Danny in the original series. In a fanfiction about betrayal, she might be pushed over the edge by jealousy or a misunderstanding about Danny's powers and his intentions. She could then use her ghost - hunting skills against Danny in a more malicious way than before.
A new character could be introduced as the betrayer. This character could be a fellow robot created by the same technology as Mega Man but with a corrupted program. This character could pretend to be on Mega Man's side, gain his trust, and then stab him in the back when he least expects it.
It could be Sakura. Maybe she becomes envious of Naruto's newfound godlike status. She might feel that he has changed too much and is no longer the Naruto she knew. So, she could side with those who want to bring Naruto down, either out of jealousy or a misguided sense of what's right for the village.
A love interest is also a common choice. In fanfictions, love can be a complicated and often treacherous thing. The love interest could have their own motives, like being forced by an external power or having a hidden agenda that conflicts with Fem Robin's goals.
It could be Vegeta. Maybe in this fanfiction, his pride gets the better of him and he feels overshadowed by Goku and Gohan's power. So he fabricates some lies to make them look like traitors to the rest of the group.