In many fanfictions, Severus Snape could be a character who betrays Fem Harry. Despite his complex relationship with the original Harry, in a fem Harry scenario, his loyalty could be swayed. Maybe he has some hidden motives or is pressured by other dark forces. Also, some minor characters like Crabbe or Goyle might be used as tools to betray Fem Harry. They're not very bright and could be easily manipulated into doing something bad to her.
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 character who might betray Fem Harry is Dumbledore. In some fanfictions, Dumbledore's actions are re - examined, and he could be seen as having his own agenda. If Fem Harry's actions or existence start to interfere with his grand plans for the wizarding world, he could choose to betray her. Additionally, new characters introduced in the fanfiction, perhaps a mysterious transfer student who has their own secrets and motives, could also be the ones to betray Fem Harry.