There could be new characters as well. Maybe some anti - hero type characters who are intrigued by dark Natsu. They could either try to manipulate him for their own gains or actually form a bond with him. On the other hand, some of Fairy Tail's older members like Makarov might also try to reach out to Natsu in their own ways, whether through force or persuasion.
Erza would definitely be involved. She has a strong sense of justice and loyalty to Fairy Tail. She'd see it as her duty to bring Natsu back and might have to fight him if he refuses. Also, Lucy could play a role. As Natsu's friend, she'd be heartbroken when he leaves and would do anything to get him back.
Gray could be one. Since he and Natsu have a long - standing rivalry, Gray might try to bring Natsu back to Fairy Tail. He'd be conflicted between his annoyance with Natsu and his concern for his former guildmate.
Gray would be one of the main characters to interact with Natsu. Their friendly rivalry is well - known in Fairy Tail. Makarov, as the guild master, would also have an important role in interacting with Natsu. He might give Natsu some important tasks or just have a grandfatherly chat with him about his return.
One possible plotline could be Natsu turning dark due to some external dark magic influence and his journey to regain his former self while away from Fairy Tail. Maybe he teams up with some unexpected allies.
Gray would be shocked at first. He and Natsu have a long - standing rivalry - but also a deep friendship. He might initially be angry that Natsu would just leave like that, but then he would realize how much he'll miss him. He might also feel a sense of loss for the guild as a whole, since Natsu is such an important part of it.
Definitely Lucy. Their bond is so strong that if there were any cracks in it, like if Lucy started distancing herself from Natsu without a clear reason, it would be devastating for him. Gray is also a candidate. They've been through so much together, and if Gray were to say or do something really hurtful, like blaming Natsu for a big mistake that wasn't his fault, it could lead to heartbreak. Juvia might be involved too. If she tried to keep Lucy away from Natsu for her own love for Gray, it could cause problems for Natsu and lead to his heartbreak.
Lucy is usually one of the main female characters. She has a deep bond with Natsu through their adventures together.
There could be a mysterious old wizard who takes Natsu under his wing when he leaves Fairy Tail. This wizard might have knowledge of ancient magics that Natsu is eager to learn.
Obviously, Natsu would be one of the main characters since the fanfiction is centered around him fighting. His friends from Fairy Tail like Lucy, Gray, and Erza are also very likely to be main characters. They would be the ones watching Natsu fight and having reactions to his actions in the battle.
It could be a group of rogue mages who have a vendetta against Natsu and Erza. They see the two leaving Fairy Tail as an opportunity to attack them when they are more vulnerable outside the protection of the guild.
Erza is frequently paired with Natsu in these fanfictions. Given her strong and leadership - like qualities, she can take charge in agent - related situations, and Natsu can be the more impulsive force. Their interactions can range from Erza scolding Natsu for his reckless actions to them having deep conversations about their responsibilities as agents. Wendy is also a character that can be paired with Natsu. Her healing magic can be crucial during dangerous agent missions, and she looks up to Natsu, which can create a sweet and protective relationship in the fanfiction.
In some fanfictions, Natsu could be framed by a dark guild as a traitor to Fairy Tail. The dark guild spreads false rumors and evidence, causing Fairy Tail to turn against him. Natsu then has to prove his innocence while also taking on the dark guild that started it all. This would lead to a lot of internal conflict within Fairy Tail and also intense battles between Natsu and the dark guild members.