One common theme is adventure. Given the nature of the Blue Exorcist world, many fanfictions will have Rin and Shirou going on various adventures together, fighting demons and solving mysteries.
One common theme is the power struggle. Given that Rin is a powerful exorcist and Amaimon is a demon, there's often a tug - of - war between their powers and how they deal with it in their relationship. Another theme could be the exploration of their different natures, with Rin being half - human and half - demon and Amaimon being a full - fledged demon.
One common theme is the growth of their relationship. Since Rin is a demon - human hybrid and Shiemi has her own insecurities, their relationship development is often explored. For example, how they learn to trust each other more deeply.
Love and friendship are also big themes. The relationship between Rin and the OC can range from a slow - burning friendship that turns into love, to an instant attraction. In many stories, they support each other through difficult times, like when facing powerful demons or dealing with the politics within the exorcist community.
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.
One common theme is innocence. Since it's about 'baby Rin', it often shows his pure and naive side before he fully understands the weight of his demon heritage. Another theme is friendship. It might explore how Rin forms his first friendships at the academy.
The theme of Rin's training as an exorcist is quite common. Fans like to write about how he learns new skills, masters his powers, and becomes a better exorcist. It could be about a particular difficult training session he has to go through, or how he discovers new ways to use his abilities in the fight against demons.
The theme of forgiveness could be explored. Maybe Shima makes a mistake that could put them in danger, and Rin has to decide whether to forgive him. Or vice versa. Additionally, their shared dreams of making the world a safer place from demons can be a central theme. They could work towards this goal in different ways and support each other along the journey.
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.
You can try websites like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, including Blue Exorcist. Just search for 'Rin x Shirou' and you'll likely find many interesting stories.