Who could be the main antagonist in 'harry potter's brother is an idiot fanfiction' if the brother is the main source of trouble?

An ancient dark magic entity that has a vendetta against the Potter bloodline. It targets Harry and James, and they have to uncover the secrets of their family's past to defeat it. This antagonist could bring in a lot of mystery and exploration of the Potter family history.

In this fanfiction, the brother being an idiot might mean he has no sense of self - preservation. He could be the type to run towards a dangerous magical creature instead of away from it, thinking it's just a friendly beast. Harry then has to constantly save him, and it becomes a running joke in the story about how clueless the brother is.

It could be a powerful ninja - demon hybrid. This antagonist could be jealous of the bond between Naruto and Inuyasha. He wants to use their combined power for his own evil purposes. So, he tries to turn them against each other first by spreading false rumors and then by attacking their loved ones.

It could be a powerful ninja who wants to capture the fox brother for his own purposes. Maybe he believes that the fox brother's powers can be used to take over the ninja world. This antagonist might be from a rival village or a secret organization within Naruto's own village.

A powerful sorcerer who wants to steal the brother's space powers for himself could be the main antagonist. He could be an ancient being who has been waiting for the right moment to take over the demigod world, and he sees the brother's powers as the key to achieving his goal.

It could be a rogue Death Eater who survived Voldemort's fall and blames Harry and his sister for the downfall of their cause. This Death Eater might be more cunning and powerful than the others, and he targets Harry's sister specifically because she represents a new threat to their dark plans. He might try to use her as a pawn to draw Harry out and then destroy them both.

It could be another powerful demon who sees the twin brother as a threat. Since he's new to the demon world, this more experienced demon might want to eliminate him.

A power - hungry politician in Nero's world could be the main antagonist. They see Nero as a threat to their power or as an easy target to gain more influence. They manipulate situations to make it seem like Nero has done something wrong.

Perhaps a long - lost relative of the Potter family who feels they were unjustly excluded from inheriting the manor. This relative could be jealous of Harry's status and the wealth associated with the manor. They might use dark magic to try and claim what they think is rightfully theirs, leading to a battle of magic and loyalty within the Potter family and among Harry's friends.

A new dark wizard or witch could be created specifically for this fanfiction. This character could be interested in using the Huntsman's skills for their own evil purposes. If the Huntsman refuses, they become enemies. This antagonist could have powers related to dark forest magic, something that the Huntsman is familiar with but in a more malevolent form.