Hermione could be the rescuer. Maybe she discovers through her research that Harry is in a bad situation at the Dursleys' and decides to take action. She uses her intelligence and magic skills to break into the Dursleys' house and get Harry out. Then they would go on an adventure together, and Hermione would be like a mentor to Harry in a way, teaching him things she has learned from books.
Dumbledore himself could be the one to rescue Harry. He might realize that leaving Harry with the Dursleys for too long is a mistake. So he shows up at the Dursleys' door one day and simply takes Harry away. Dumbledore could then take Harry on a journey to different magical places, teaching him about the history and secrets of the wizarding world, which would make Harry much more knowledgeable and confident when he finally goes to Hogwarts.
It could be an old wizard who has been living in seclusion near the Dursleys' neighborhood. This wizard senses Harry's magic and the Dursleys' cruelty. He comes to the rescue, taking Harry to his small but magical cottage. There, the wizard teaches Harry about the ancient magic that has been forgotten by most of the wizarding world. Harry then grows up with a different perspective on magic and becomes a very powerful wizard by the time he reaches Hogwarts.
It could be Dumbledore. He might have realized that the Dursleys were not the best place for Harry after all and decided to take matters into his own hands, using his powerful magic to rescue baby Harry.
In this kind of fanfiction, perhaps Harry is rescued by the Order of the Phoenix earlier than in the original story. They might have realized the Dursleys' mistreatment sooner. Then Harry could start his magical training earlier, away from the hostile environment of the Dursleys' home. He may also form closer bonds with other members of the Order, like Sirius, who could play a more active role in his upbringing.
Perhaps Harry is rescued by an unexpected character, like a centaur who has been secretly watching over him. The centaur arrives at the Dursleys' on a moonlit night, and using its natural magic and knowledge of the stars, it convinces Harry to come with it to a safe place in the Forbidden Forest where he can be protected and trained.
It could be the Queen of England, as in the real - world monarchy. In the fanfiction, she might be a figure with hidden knowledge of the wizarding world and decides to use her power to rescue Harry.
Dumbledore might play a role. He could be the one who encourages Harry to stand up to the Dursleys. He might have some hidden knowledge about the relationship between the wizarding world and the Dursleys that could help Harry. Also, Hagrid could be involved. He could provide some muscle if things get physical. He might also offer some emotional support to Harry, as he's always been like a big, friendly giant to him.
Dumbledore could rescue Harry. He's always been watching over Harry and would surely sense if the Dursleys had abandoned him. Dumbledore has the power and wisdom to take Harry to a safe place, perhaps a different magical family or a special hideout. He could also start teaching Harry about his past and his parents earlier, which would give Harry a better understanding of his place in the wizarding world.
One possible scenario could be that a powerful wizard or witch, perhaps someone who knew James and Lily Potter well, comes to rescue baby Harry. They might use magic to bypass the Dursleys' defenses and take Harry away to a safer place, like a hidden wizarding community.
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.
Hermione Granger could also be a rescuer. She was always intelligent and resourceful. In a fanfiction, she might figure out that Harry was in danger at the Dursleys' through some research or a hunch. Then she could use her knowledge of magic to break into the Dursleys' house and get Harry out. She would be brave enough to take on such a task to save her friend.
Well, Harry Potter is surely one of the main characters since the title implies it's about him being taken from the Dursleys. There might also be some original characters created by the fanfiction writer to play important roles in Harry's new journey.