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.
There are many interesting fanfictions centered around Rin's tail collar in Blue Exorcist. Some might explore the significance of the collar in his journey as an exorcist. It could be a symbol of his demon side that he constantly has to manage, and fanfictions may delve into how he feels about it on a deeper level.
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 effective way is to show Rin's loss of interest in things he used to enjoy. For instance, if he used to love training or cooking, suddenly he has no enthusiasm for it. His appearance can also change. He might look disheveled, with messy hair and dark circles under his eyes. Writers can also use flashbacks to his happier times and then contrast them with his current depressed state. By showing how much he has changed, it emphasizes the depth of his depression. And his reactions to small things can be exaggerated. A minor setback might make him feel completely defeated, which is a sign of his fragile mental state.
Well, it could be a story where Rin and Shiemi are on a mission together. As they face a powerful demon, Shiemi gets injured. Rin, worried sick, nurses her back to health. In a moment of relief and new - found closeness after the ordeal, they share a kiss. It's a sign of their growing bond and the start of a deeper relationship.
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.
The story would change in terms of the romantic sub - plots. If Rin was a girl, there could be new love interests emerging. Maybe some characters who were just friends before would start to have romantic feelings for her. For example, Bon might see her in a completely different light and develop a crush.
It could be that the white hair is a symbol of some special power or transformation Rin undergoes in the fan fiction. Or, the author wanted to make Rin stand out more visually.
It could end happily with them becoming a couple and continuing to fight demons together. They support each other in their exorcist duties and their relationship grows stronger over time.
Well, 'Destined to be Together' is a great one too. The author does an excellent job of portraying the OC's character. She has her own insecurities and strengths, and as she gets closer to Rin, they both learn and grow. The story also has some great action scenes related to exorcising demons, which adds more excitement to the overall plot.