They could be some kind of old and wise wizards. Maybe they are a group that has been secluded for a long time, preserving ancient magic.
In this fanfiction, the greybeards could be a collection of elder wizards who have dedicated their lives to the study and preservation of magic. They may have their own unique traditions and ways of teaching. They might be the keepers of magic that is considered too dangerous or too powerful for the general wizarding population. By training Harry, they are passing on this knowledge to a new generation, hoping that he will use it for good and not let it fall into the wrong hands.
The greybeards might be an order of very old wizards who have lived through many eras of the wizarding world. They possess a vast amount of knowledge and experience. They might be known for their long beards which are a symbol of their age and wisdom. They choose to train Harry perhaps because they see in him the potential to bring about some great change in the wizarding world or to combat a new and emerging threat.
Well, without reading it specifically, it might be about Harry Potter getting trained by greybeards. Maybe these greybeards are some kind of wise old wizards who impart special knowledge or skills to Harry that he didn't get at Hogwarts. It could involve new forms of magic, different ways of dealing with dark forces, or perhaps a whole new adventure that takes Harry outside the normal realm of the wizarding world he knows.
Zelretch is likely a very powerful and perhaps mysterious character in this fanfiction. He could be an ancient wizard or a being from a different magical dimension with knowledge far beyond what is known in the normal Harry Potter universe.
Azura could be an original character created for this fanfiction. Maybe she's a powerful witch from a far - off land with her own unique brand of magic.
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.
The main characters would be Harry Potter and Batman. Harry Potter, the young wizard from the wizarding world, is the focus as he is being trained. Batman, the Dark Knight from Gotham, brings his skills and knowledge to the table. There might also be some appearances from other characters in the Harry Potter universe, like Hermione and Ron, who could react to Harry's new training. And in the Batman side, perhaps Alfred could also play a role, providing support or advice in the training process.
The main characters are Harry Potter and Thor. But it's possible that some other important characters from both universes could play a role. In the Harry Potter universe, Dumbledore might have an opinion on Harry's new training. And in the Thor universe, Odin could be aware of Thor training an outsider like Harry. Their reactions and interactions could add more depth to the story in this fanfiction.
Obviously, Harry Potter and Godric Gryffindor. There might also be some other Hogwarts founders making cameo appearances.
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.
It's hard to say for sure without reading the fanfiction. But it could be the usual suspects like Voldemort and his Death Eaters. However, with Kratos in the picture, there might be new antagonists from Kratos' world who see Harry as a threat now that he's being trained by such a powerful being.