There's a chance that Scar could betray Edward in certain fanfiction concepts. Scar has his own motives and a complex past. If he feels that Edward is getting in the way of his revenge or his understanding of alchemy, he might betray him. Additionally, some fanfictions might explore the idea of an old acquaintance from Edward's childhood betraying him. Maybe someone who was once a friend but has since been corrupted by power or a dark influence. And then there's always the possibility of a new, mysterious character being introduced specifically to betray Edward for some yet - to - be - revealed reason.
One likely character could be a rival alchemist who is jealous of Edward's skills and reputation. Another might be a villain from the original story who has a grudge against Edward. Also, a new character created specifically for the fanfiction, like a power - hungry alchemist who thinks Edward can help him achieve his goals.
Alphonse Elric is often present. He might play a role in helping Edward understand and use his new gamer - like abilities in relation to alchemy. For example, he could be the one who discovers a new aspect of Edward's powers.
Rias could be a character who betrays Issei in some fanfictions. If she were to face a difficult decision between protecting her family's reputation and standing by Issei, she might choose the former. Another possibility is Akeno. If she were to be influenced by some ancient evil power that made her turn on her friends, Issei could be a target.
Natsu could be a character who betrays Lucy in some fanfictions. Maybe he gets influenced by some dark magic that makes him turn on her. Gray is also a possibility. He might be jealous of Lucy's growing power or popularity in the guild and decides to betray her for his own gain.
Vegeta could be a likely character. In some fanfictions, his pride and envy of Goku's strength might lead him to betray Goku. He might think that by getting rid of Goku, he can become the strongest in the universe. Another one could be Frieza. Even if he pretends to be an ally in some alternate storylines, he could betray Goku at a crucial moment to regain his power and dominance. Also, a new and powerful character introduced in the fanfiction, who is envious of Goku's popularity and power, could easily be a betrayer.
Obviously, Edward Elric himself is a very popular character. He's the protagonist of the original story and his return is the central theme, so he's always in the spotlight in these fanfictions.
In certain fanfictions, a Pokémon professor or an authority figure in the Pokémon world could betray Ash. They may have their own secret agendas. For example, a professor might use Ash's research data without his permission or manipulate him into dangerous situations for their own experiments. This would make Ash feel extremely betrayed as he looks up to these figures for guidance.
In fanfiction, Gary could be a likely character to betray Ash. He has always been a rival and might have ulterior motives. For Dawn, Zoey could potentially betray her. Zoey has been a tough competitor and might be jealous enough to take some underhanded actions. Another possible betrayer for Ash could be Team Rocket. They've been after his Pikachu for a long time and could use betrayal as a new tactic. For Dawn, perhaps a new coordinator who is envious of her popularity in the coordinator world.
Edward might meet his parents. Since he has the chance to go back in time, he could meet them when they were alive and perhaps learn more about alchemy from them, which could change his future actions.
Sasuke is a likely candidate in some fanfictions. Given their complex relationship throughout the original series, it's easy to imagine a scenario where Sasuke's thirst for power or some misinformation leads him to betray Naruto. Another could be Sakura. If she becomes jealous of Naruto's growing power or if she's manipulated by outside forces, she might turn against him.