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.
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.
It could be a jealous rival from another guild. They might see Natsu as a threat and figure framing him would be a good way to get him out of the way.
It could be Issei. Maybe he becomes jealous of Natsu's power or popularity in DxD High School and decides to betray him for his own gain.
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.
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.
One of the senior and more experienced members of the guild. They have seen a lot and know how things should be done in the guild. If Natsu acts in a way that goes against their long - established ways, they would not hesitate to berate him.
It could be a young, inexperienced mage who has always admired Natsu from afar. Maybe someone with a hidden power that Natsu senses and decides to take under his wing.
It could be a dark wizard who has a vendetta against Fairy Tail. Maybe someone who Natsu defeated in the past and is now seeking revenge.
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.
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.