He can simply avoid reading it. Just stay away from any fanfic related to Edward and focus on the original work or other things he likes.
Redemption can also be a theme. They both have made mistakes in their journey, and fanfics may explore how they try to make up for those mistakes. They might seek forgiveness from each other or from the people they've wronged, all while strengthening their bond as brothers.
Maybe he has different views on the characters. He might think that fanfic often distorts the original image of Edward he has in mind, so he hates it.
If Ash hates Serena fanfic, perhaps the fanfic community could come up with some guidelines. These guidelines would ensure that the fanfics respect the essence of all the characters, including Ash. They could also encourage creativity in a way that doesn't make any character feel misrepresented or out of place. Additionally, the creators of the fanfic could ask for feedback from the broader Pokemon fan community to see if their work is likely to be acceptable to characters like Ash, based on the general understanding of the characters' personalities and the world they live in.
One option could be to simply avoid showing Karin any Sakura fanfic. Respect her feelings and keep that type of content away from her.
If the oc hates it but still wants to give it a chance, they could try to find a different take on the rias fanfic. Maybe look for ones that are more in line with their own values or interests. For example, if the oc is interested in action, find a rias fanfic that has more action elements rather than just the typical romance. Another thing to do is to try and write their own version of a rias - related story that they would like, incorporating the elements they enjoy and leaving out the ones they don't.
Well, she could simply avoid reading them. There are plenty of other things related to Sonic that she can engage with, like the official games, comics, or animated shows.
One characteristic is the expansion of their personalities. In fanfiction, we might see more of Edward's determination in different situations or Alphonse's kindness and wisdom. Another aspect is the world - building. Some fanfictions create new places and alchemy - related concepts around Edward and Alphonse. Also, the relationship between the two brothers is often deepened, showing their unwavering support for each other in various challenges and adventures.
Well, 'Alchemy and Brotherhood' is a must - read. It focuses on how their study of alchemy affects their relationship. The author does an excellent job of showing how they grow together in their understanding of alchemy and each other, and it's full of heartwarming moments.
The OC can simply avoid reading any more of that fanfic. There are plenty of other stories out there in different fandoms or even within the 'High School DxD' fandom that might be more to their taste.
One thing that could be done is to keep the fanfics away from him. Don't show him any new fanfics or talk about them in his presence.