At first, they might be in disbelief. The members of Fairy Tail see Natsu as a core part of the guild, so it would be hard for them to accept that he has betrayed them. They would probably try to talk to him and find out what's going on. Some members might be angry immediately, thinking that he has turned his back on their family. But others, like Lucy, might be more hesitant to believe it and would look for a way to resolve the situation without conflict.
He might initially be shocked and confused. Natsu has always been so loyal to Fairy Tail, so this would be a huge blow to him.
The guild would likely be shocked at first. They consider Natsu as one of their own, so his attack would be completely unexpected. Some members might try to talk him down, like Lucy. She has a close bond with Natsu and would probably try to reason with him.
The guild members' reactions would vary greatly. Happy, being Natsu's closest companion, would likely be excited about the fanfiction. He might fly around the guild hall, telling everyone about it and adding his own little comments about how accurate or inaccurate it is. He would definitely be protective of Natsu's image in the fanfiction, especially if it's not a positive portrayal.
The guild would likely be ecstatic. Gray would probably make some snarky comment about Natsu finally growing up, but deep down he'd be happy for his friend. Erza would be all for it and might start planning the wedding immediately, giving orders to everyone in the guild to help out.
Emotionally, Natsu could become more empathetic. If he meets characters in the dark guild who had a difficult past that led them to the dark side, he might start to see things from their perspective. This doesn't mean he would support their actions, but he would understand them better. For example, if there's a character in the dark guild who was orphaned and turned to dark magic for survival, Natsu might feel sympathy for them and try to offer an alternative path rather than just attacking them. This would show a more mature and compassionate side of his character that wasn't as prominent before.
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.
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.
She would fit in very well. The Fairy Tail guild is all about family and acceptance. Given her parents are Lucy and Natsu, she'd be welcomed with open arms. She could learn from the older members like Makarov and Mirajane, and bond with the younger ones like Romeo. Her magic skills would also contribute to the guild's strength during battles or when helping the town.
Some of the more mischievous guild members like Gray might tease them a lot. He'd probably make jokes about it every chance he gets, much to Natsu and Lucy's annoyance. Happy, on the other hand, might be really confused at first but then start to see it as a sign of their closeness and be really happy for them.
Some members might be in shock. For example, Happy, who is Natsu's closest friend, would be extremely sad and might even consider leaving the guild too. He has always been by Natsu's side, and the sudden expulsion would leave him feeling lost.