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.
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.
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.
Yes, in a lot of Fairy Tail fanfiction, Natsu can be overpowered. Writers often like to amp up his powers for more exciting storylines. For example, they might give him new fire - based abilities or make his existing ones even stronger to defeat more powerful foes.
In fanfiction, Natsu's age can vary greatly. Some authors might keep him at his canonical age from the original Fairy Tail series, which is around 17 - 18 years old at the start. However, in alternate universe fanfictions, he could be older or younger depending on the plot. For example, in a time - travel based fanfic, he could be much older if the story takes place far into the future or much younger if it goes back to his childhood days.
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.
Obviously, Natsu is a very popular character as it's in the title. He is often the center of many fanfictions. Lucy is also quite popular as she has a close relationship with Natsu in the original series. Their dynamic often plays a big part in fanfictions.
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.
Goku from Dragon Ball Z could be a great crossover character for Natsu. They are both very strong and have a love for fighting. Goku's ki - based attacks and Natsu's fire magic would make for some epic battles. They could also train together and learn from each other's fighting styles. Also, Alucard from Hellsing could be an interesting crossover. His vampiric powers against Natsu's magic would create a very dark and intense story.
In a crossover with 'Naruto', Naruto himself would be a great character to pair with Natsu. They both have strong personalities and are determined to protect their friends. They could learn from each other's fighting styles, with Naruto showing Natsu some of his ninja techniques and Natsu teaching Naruto about magic.