Some fanfics focus on Rin's power development in a different way from the original story. Maybe she discovers new powers or has to learn to control her existing ones in a more challenging situation. There are also fanfics that explore Rin's family relationships in more depth, especially her connection to her father. This could involve flashbacks or new revelations about her family's past.
One possibility could be that his relationships with other characters would change drastically. For example, his friendship with Yukio might become more protective on Yukio's part as he now has a sister - like figure in Rin. Also, his interactions with the other exorcist trainees could be different. They might be more hesitant around him as a girl, or some might be overly friendly in a different way.
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.
In a fanfic where Rin is secretly a girl, the author could explore themes of identity and self - discovery. Rin might struggle with coming to terms with her true gender while also dealing with the challenges of being an exorcist. It could add a whole new layer of depth to the character and make for a very engaging story.
Perhaps in the fanfic, Rin's death could be a result of a curse. He might be tricked into a situation where he's cursed, and no matter what he does, the curse slowly drains his life. As he nears death, he could be reflecting on his relationships, his journey as an exorcist, and all the things he didn't get to do. His friends would then be on a desperate quest to find a way to break the curse, but they're too late.
It could be about Rin being bound to a powerful demon or exorcist as a familiar. Maybe he has to follow their commands and use his powers in ways he didn't expect. For example, he might be forced to fight other exorcists or demons on behalf of his 'master'.
One common plot twist could be that Rin, as a girl, has a different approach to her exorcist training. Maybe she uses her femininity to her advantage in a way a male Rin couldn't. For example, she might be more perceptive to the emotional states of the demons she encounters, which could lead to either more successful exorcisms or getting into more dangerous situations because she gets too involved.
Well, one thing that could happen is that her relationships with other characters would shift. For instance, some male characters who were just friends might start having different feelings towards her. And she might face different challenges from other female characters who could be jealous or supportive in different ways.
There are several themes that are quite popular. One is the idea of redemption. Since Rin has the demon blood in him, some fanfics might explore how Shura could be involved in his journey towards redemption, or how their relationship is affected by Rin's struggle with his demon side. Another theme is the clash of cultures. Rin grew up in a different environment compared to Shura, and this can lead to interesting scenarios in fanfics where they have to bridge the gap between their different ways of thinking and living. And then there's the theme of secrecy. Their relationship might need to be kept a secret for various reasons in the 'Blue Exorcist' universe, and fanfics often play with this concept, creating a sense of mystery and tension in the story.
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 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.