The most common theme is probably the battle between good and evil within them. The Okumura triplets are in a unique position as they are related to demons but are exorcists. Fanfics might show how they fight the inner demons, both literally and figuratively. Love and romance can also be a theme. They could fall in love with someone who either accepts or challenges their dual nature. Additionally, themes of redemption and sacrifice are often seen. For example, one of the triplets might sacrifice something for the sake of the others or to save the world from the demonic threats.
Rin Okumura's friends like Shiemi Moriyama are quite popular. She has a gentle and kind personality, and her relationship with Rin is often explored in different ways in the fanfics. Another character is Yukio Okumura. Even though he is one of the triplets, his role as an instructor and his complex relationship with his brothers make him stand out more in some fanfics. Bon is also a favorite. His strong - willed and sometimes hot - headed nature adds an interesting dynamic to the stories.
The idea of the triplets having to face a powerful and mysterious enemy together is also quite common. It adds an element of adventure and danger. And of course, there are themes related to their family background, like how their heritage as the sons of Satan affects their lives and relationships within the exorcist community.
One common theme is the exploration of Satan's past. Writers often try to fill in the gaps in the original story about his history and motives.
In 'Blue Exorcist Rin fanfic', a very frequent theme is Rin's battle against his inner demons, both literally and figuratively. His journey to master his demonic powers while still remaining true to his human side is often explored. Also, his relationship with the exorcist community, whether it's facing discrimination or finding acceptance among them, is a common theme. Additionally, his friendships, especially with the people he meets at True Cross Academy, are often a big part of the stories.
Many fanfics also focus on her powers and how she learns to control and use them. It's about her journey of becoming stronger both physically and mentally.
One interesting Blue Exorcist fanfic could be about an alternate universe where Rin and Yukio have different powers. Maybe Rin has the power to control time instead of blue flames, which would completely change the story's dynamics and the relationships between characters.
One common theme is Rin's struggle with his identity as a half - demon. In many fanfics, his depression stems from feeling like he doesn't belong in either the human or demon world. Another theme could be his relationship with his brother. If there are unresolved issues or misunderstandings between them, it can add to his feelings of depression. Also, the fear of his demonic powers getting out of control and harming those he loves is often explored, which can lead to his depressed state as he tries to keep his powers in check while also living a somewhat normal life.
One common theme is friendship. For example, the bond between Rin and his friends at the True Cross Academy is often explored in fanfics. Another theme is the battle against demons. Fanfic authors may create new scenarios where the exorcists face more powerful demons.
One theme that pops up a lot is the struggle with their exorcist duties while also dealing with their feelings for each other. They have to fight demons and protect the world, but at the same time, they are developing these complex emotions. It's like a balance between their personal lives and their responsibilities. And also, the theme of acceptance is there. They have to accept each other's flaws and strengths as they get closer.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). There are often many talented writers who create fanfiction about the male triplets in Blue Exorcist. Another option is Fanfiction.net. Just search for 'Blue Exorcist male triplet' and you'll get a list of relevant stories.