Maybe it could be a new character introduced in the fanfiction. This character could be an outcast from a different magical community who has a natural inclination towards combat. Harry Potter, seeing their potential and the need for a strong defender, decides to train them to be a killer. They would bring a fresh perspective and new skills to the story.
Neville Longbottom could be a likely candidate. He has shown great courage and has been through a lot. If the situation was dire enough, Harry might train him to be a more effective fighter, bordering on a killer, to protect Hogwarts and the wizarding world.
Hermione Granger could also be a possibility in a different take on the fanfiction. While she is known for her intelligence and rule - following, in a situation where the rules no longer apply and the wizarding world is in extreme danger, Harry might train her to be a killer. She would use her intelligence to strategize her kills in a way that is efficient and less likely to be detected. However, this would also create a great internal conflict within her as it goes against her moral code.
In the 'Harry Potter Trained Killer' fanfiction, the plot could revolve around a time - traveling scenario. Harry Potter, having seen the future destruction of the wizarding world, decides to train a person from the past to be a killer. This person then has to go through a series of adventures and challenges. They have to learn combat skills from Harry, as well as how to be stealthy. Along the way, they discover secrets about the wizarding world that even Harry didn't know, and they have to use their new - found killer skills to protect these secrets from falling into the wrong hands.
Obviously, Harry Potter and Godric Gryffindor. There might also be some other Hogwarts founders making cameo appearances.
Obviously, Harry Potter and Flitwick are the main ones. Harry as the student being trained and Flitwick as the trainer.
Obviously, Harry Potter would be one of the main characters. And then, there would probably be some members of the SAS. They could be tough, experienced soldiers who take on the task of training Harry.
I think Harry and Blake could also be paired well. Blake's intelligence and her past experiences with the White Fang could be complemented by Harry's knowledge of fighting dark forces. They could bond over their shared experiences of fighting against those who want to bring harm and chaos, whether it's the Grimm in Remnant or Voldemort in the wizarding world.
A new, unknown villain. Maybe a brilliant scientist who turns to the dark side after a personal tragedy. This villain could create a weapon specifically designed to counter Batman's abilities, like a suit that can absorb all of Batman's punches and then redirect the force back at him, ultimately killing him.
Obviously, Harry Potter himself. And then there would be the goblins. Maybe Griphook, as he was one of the more well - known goblins in the original series. He could be one of the main trainers of Harry.
It could be a deranged fan who has found a way to bring their dark fanfiction ideas to life. This person might be jealous of the original story and wants to create chaos in the wizarding world through their fictional concepts made real.
There's a chance it could be a powerful house - elf. House - elves are known for their own brand of magic, and one particularly rebellious or enlightened house - elf could decide to train Harry in magic that is more about stealth and quick - thinking. After all, house - elves have to be very sneaky to avoid detection by their masters most of the time, and this kind of skill could be very useful for Harry in his battles against Voldemort.
Maybe it's a former Death Eater who has seen the error of his ways. He wants to atone for his past sins by training Harry to defeat Voldemort. He has inside knowledge of the dark side's tactics and uses this to make Harry a more formidable opponent.