Dobby the house - elf could also be an ally. He has a soft spot for Harry and has helped him before. He could use his house - elf magic, which is different from wizard magic, to get Harry out of the Dursleys' place. Maybe he could apparate Harry away when the Dursleys are not looking, or use his magic to create distractions so that Harry can escape on his own.
The Weasley family, especially Fred and George. They are known for their mischievous and creative minds. They might come up with some unique and magical ways to break Harry out of the Dursleys' imprisonment. For example, they could create some sort of diversion or a magical device that can unlock the doors or bypass the Dursleys' security measures.
Hedwig could be a potential ally. She could carry messages to Harry's friends at Hogwarts, alerting them of his situation. Even though she is just an owl, she has shown loyalty and intelligence in the original series.
Some potential new allies could be the house - elves. They might see Harry's plight and decide to help him, using their unique magic and knowledge of Hogwarts' secrets. Another could be the centaurs in the Forbidden Forest. They are very wise and may take Harry under their wing.
His Hogwarts friends could be his allies. Ron and Hermione might come up with a clever plan to get Harry out. They could use the Weasley's magic - enhanced car to fly to Privet Drive and then create a diversion while Harry sneaks out. Another ally could be Hagrid. He has a soft spot for Harry and might show up with a large magical creature, like a friendly hippogriff, that Harry can ride away on.
There could be a new character, a powerful and kind - hearted witch or wizard who has a premonition about Harry's future. They might decide to step in and save him from the Dursleys. This new character could bring a fresh perspective and new magical abilities to the story as they rescue Harry and take him under their wing.
Dumbledore could be an unexpected ally. He might see this as an opportunity for Harry to stand up for himself and gain some closure on his difficult childhood. Also, some of the Hogwarts professors, like McGonagall, could be on Harry's side. They have always cared for Harry's well - being and would not tolerate the Dursleys' abuse. And there could be some new magical characters introduced specifically to help Harry in his legal battle, like a wizard lawyer who has a soft spot for underdog cases.
Hermione could be one. Given her open - mindedness and thirst for knowledge, she might be intrigued by the Demons Child and see its potential for good rather than just being scared of it.
Hermione would definitely be one of his allies. She is smart and would be able to help him research about his new identity and how to control his powers. For example, she could find ancient texts in the Hogwarts library that relate to both the wizarding world and the Greek gods.
His friends from Hogwarts could still be his allies. Hermione would use her intelligence to help him understand his new situation. Ron would be there for moral support. Additionally, any friends or allies that Rias has in her world could also become Harry's allies. They might be magical creatures or other powerful mages.
Hermione would definitely be an ally. She's smart and would be able to help Harry research about his 'son of Mars' heritage, finding ancient spells or texts that could explain it further.
In this fanfiction, some of the potential allies for Harry could be the magical creatures that have a connection to Norse mythology. For example, the Valkyries. They could help Harry in battles or in understanding his new heritage. Another ally could be Odin, Frigga's husband. Despite the initial shock of Harry's existence, he might see the value in Harry and offer his protection and guidance. And don't forget Neville. He's always been a loyal friend to Harry, and in this new scenario, he would continue to be an ally, perhaps even more important as Harry faces new and different challenges.
One potential ally could be Hagrid. Hagrid has always been a friend to Harry, and in this fanfiction, he could help Harry understand his newfound heritage as the son of Hephaestus. Hagrid's knowledge of magical creatures could also be useful in Harry's adventures related to his Hephaestus - given abilities.