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.
Happy, Natsu's little blue Exceed friend, is also popular in fanfictions. He can add a touch of humor and often provides some comic relief. In some fanfictions, he might have his own little adventures or be the key to helping Natsu in a difficult situation, even if it's just by providing some much - needed moral support.
Gray is another popular character. His cool - headedness (most of the time) and his unique ice - magic contrasts well with Natsu's fiery nature. In fanfictions, their friendly rivalries can be explored in new and interesting ways. Erza is also a fan favorite. Her strong - willed and powerful personality makes her a great addition to any fanfiction plot, whether it's as a supporting character or taking on a major role.
In some fanfictions, Natsu as the 1st master of Fairy Tail might be depicted as a powerful and visionary leader. He could be shown as someone who laid the foundation for the guild's values of friendship and strength.
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.
Obviously, Natsu himself is a central character. He is the main attraction in the harem scenario. His fiery personality and powerful magic make him a key figure. Then there's Lucy. She is often included in the harem as she has a strong bond with Natsu in the original Fairy Tail series. Her celestial spirit magic also adds an interesting element to the fanfiction. Happy, the blue flying cat, is also popular. He can provide comic relief and also plays a role in the adventures, especially when it comes to helping the children in the story.
A character from a different guild who is on the run from some dark forces. Natsu could offer protection and in the process, this character could introduce new fighting styles and magic types to Fairy Tail. Also, a magical creature that can talk and has the power to sense hidden magic could be added. Natsu could befriend this creature and it could help him in his quests.
Perhaps as the Fourth Master, Natsu discovers some long - lost secrets about Fairy Tail. These secrets could be related to the origin of the guild or some powerful magic that was hidden away. He then has to figure out how to use this knowledge to the guild's advantage without causing chaos.
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.
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.
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.