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.
Juvia is likely to interact with Natsu too. She might be a bit jealous seeing Natsu's closeness with Lucy. And then there's Wendy, who could look up to Natsu as a big brother figure. Their interactions could range from Wendy asking for advice on her magic to Natsu simply giving her words of encouragement on her growth as a mage.
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.
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.
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.
Gray would also be important. There could be a rivalry - turned - friendship aspect where Gray, who has his own ice - making powers, and Natsu, as a dragon, have to learn to work together. Maybe they compete in a friendly way to see whose powers are more useful in different situations.
Lucy is often paired with Natsu in many fanfictions. Their relationship can be explored in different ways in these fan - created stories. It could be a more romantic relationship than in the original series, or they could have a different kind of partnership, like being more equal in power or having different goals they pursue together.
It could be a rogue dragon slayer. Someone who has turned against the ways of dragon slayers and wants to destroy Fairy Tail for some personal vendetta against Natsu or the guild as a whole.
Well, Lucy would be likely to interact with Naruto. She's friendly and curious, so she'd probably be one of the first to approach Naruto and try to understand where he came from. Gray is also a candidate. His cool - headedness (pun intended) would contrast with Naruto's more energetic personality. And of course, Makarov, the guild master. He would be interested in Naruto's abilities and might offer him a place in the guild or give him some tasks to prove himself in the Fairy Tail world.
Lucy could be a main character. She was always close to Natsu in the original Fairy Tail. So when Natsu returns as an 'ex' and meets the guild again, she would likely play a big role in how he is received and in the development of the story.
Seilah is also a main character in this context. She has her own set of powers and a rather complex personality in the Fairy Tail universe, which can be further explored in fanfiction.
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.