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.
Perhaps in the fanfiction, Natsu is betrayed by someone he trusts in a mission. For example, they might leave him behind in a dangerous situation to fend for himself while they take the credit for the mission. Or it could be that a love interest betrays Natsu by choosing someone else over him due to some false perception or external pressure.
Natsu would likely be extremely hurt. He has always been loyal to Fairy Tail, so being betrayed would be a huge blow. He might initially be in disbelief, not wanting to accept that his friends and guildmates could turn on him.
Natsu could also be deeply hurt. He values his friends highly in Fairy Tail. If betrayed, he might isolate himself, doubting his own judgment in trusting people. This could lead to a period of self - discovery where he tries to figure out how to trust again. For example, he might recall the times when his friends in Fairy Tail were truly there for him and use those memories to heal and grow stronger in the face of this betrayal.
One possible reason could be a misunderstanding. Maybe the other members of Fairy Tail misinterpret Natsu's actions as something harmful to the guild when in fact he has good intentions. For example, if Natsu accidentally destroys a valuable guild property while fighting a powerful enemy in the city, the guild might see it as irresponsible behavior and turn against him.
A more detailed title could be 'In the World of Fanfiction: Fem Natsu's Journey After Betrayal'. This title not only mentions the main idea of fem natsu being betrayed but also implies that there will be a journey or development in the fanfiction that follows the betrayal.
She might become angry at first. Natsu is known for her fiery temper. So, she could lash out at the person who betrayed her, confronting them directly and demanding an explanation.
Well, often in such a 'ash gets betrayed fanfiction', the story might start with a seemingly normal day for Ash. Then, out of the blue, someone he trusts deeply, like Pikachu or one of his human friends, does something that he perceives as a betrayal. This could be something as simple as not believing in him during a crucial battle or siding with an enemy for a reason that's not clear to Ash at first. It can create a lot of emotional turmoil for Ash and make for a very engaging read.
He could marry an original character created specifically for the fanfiction. This character could be a powerful mage from a far - off place who crosses paths with Natsu during one of his adventures.