I think it could be Neville Longbottom. Neville has grown into a strong and confident wizard. He could have discovered boxing as a form of self - defense and then decided to teach Harry. Neville would bring his own unique perspective, having been bullied in the past. He'd train Harry to be both strong and fair in the boxing ring, emphasizing the importance of honor and respect in combat.
Sirius Black could be a trainer. Sirius, being Harry's godfather, would be highly motivated to train Harry. He could use the time - turner to go back to a time when he was at his peak in terms of combat skills and pass on his knowledge to Harry, especially in the areas of dark - wizard fighting and survival skills.
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.
It could be Dumbledore. Even if he's dead in the original story, in fanfiction, anything is possible. He might have left behind some kind of magic that allows him to train Harry from beyond the grave, or he could have faked his death to take Harry to a secret location for training.
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.
Obviously, Harry Potter himself could be the main character. He has the skills and the popularity from the original series. Ron Weasley could also be a main character, maybe as Harry's trainer or his corner man in the boxing matches. Hermione might be involved as well, using her intelligence to strategize for Harry's boxing career.
In this fanfiction, Harry could be a boxer to raise money for the rebuilding of Hogwarts after the war. He travels to different wizarding communities to participate in boxing matches. Along his journey, he meets some old enemies who now also see the value in his cause. They form an unlikely alliance, and Harry's boxing skills become a symbol of unity and hope for the wizarding world.
It could be Hagrid. He has a lot of strength himself and might know some basic principles of building muscle. He could guide Harry on how to lift the heavy weights, using his own brute strength as an example.
He would probably have a very unique team. For example, he could have a Ninetales because of its magical and fox - like appearance, which might remind him of his Patronus. His training methods would likely be a mix of wizardry and traditional Pokemon training. He might use spells to heal his Pokemon or to enhance their abilities in battles.
Hermione would definitely be one of his top allies. Her intelligence and knowledge of magic and history would be invaluable in helping Harry rule. She could advise him on laws and policies, especially those related to education and the treatment of non - human magical beings. Ron would also be an ally. His loyalty and his understanding of the wizarding world's less - sophisticated side could help Harry connect with the common wizards.
Hermione would be a likely colleague. She's highly intelligent and could be teaching a subject like Ancient Runes or Arithmancy. Ron could also be there, perhaps as a flying instructor or teaching something related to magical creatures. Neville might be a Herbology professor, given his love and talent for that subject. Luna could be teaching something really unique, like the study of magical creatures that are often overlooked or misunderstood.