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.
Well, first of all, Lucy. She's Natsu's main partner in adventures, and if she suddenly disappeared from his life or if there was a love triangle situation involving her, Natsu would be heartbroken. Lisanna could also be involved. If she came back into Natsu's life in a way that complicated his feelings for others, especially if he had developed new feelings for someone else like Lucy, it could be a source of heartbreak. And then there's Makarov. He's like a father figure to Natsu. If Makarov were to expel Natsu from the guild for some unjust reason, it would shatter Natsu's heart as the guild is his home.
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.
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 is another character. His ice - magic can complement Natsu's fire - magic. They might team up to fight enemies or overcome obstacles in the way of saving Fairy Tail. Sometimes they bicker while doing so, which adds some humor to the story.
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.
Mirajane is a popular choice too. She has a mysterious side to her, and in the fanfictions, she could use her unique abilities to help Natsu or be involved in the events that the Fairy Tail members are watching. Also, Laxus might be included. His powerful magic and complex character can add depth to the fanfiction stories, especially when his relationship with Natsu and the rest of the guild is explored.
Happy is almost always there. He's Natsu's loyal sidekick and provides comic relief as well as support during training. Lucy also frequently appears. She might offer some strategic advice or just be there to cheer Natsu on. Erza is another common character. Given her strong and strict nature, she could be the one pushing Natsu to his limits during training.
Most likely, the other members of Fairy Tail would be involved. For example, Lucy might be there. She could be in distress during the attack that Natsu is trying to fend off, or she could be one of the first to find Natsu after he has fallen. Gray could also be involved, perhaps he was fighting alongside Natsu or arrives shortly after to see what has happened. Erza too, as a strong and important member of the guild, she may play a role in the events following Natsu's fall.
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.
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.
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.