First of all, Harry's closest friends, Hermione and Ron, would be right there with him through the process. They would support him but also try to understand the implications of this new magic. Voldemort and his Death Eaters would be greatly affected. They would see Harry as an even greater threat and might change their strategies to deal with him. The magical creatures in the Forbidden Forest, such as the centaurs and the hippogriffs, might also sense the change in Harry's magic and react differently to him. They could either be more friendly or more cautious, depending on how they perceive his new power.
His friends would surely be affected. Hermione and Ron might be both amazed and a bit worried. They could be worried that this new power might change Harry too much or put him in more danger. Also, his enemies like Draco Malfoy would be affected. Draco might become more jealous or even more afraid of Harry.
His friends would surely be affected. Ron and Hermione might be in awe of his newfound powers. They could also benefit from his abilities. For instance, Harry could use his time - traveling ability to save Hermione from the time - turner accident. And Ron could get some special training from Harry to become a much more powerful wizard himself.
When raised by an Acromantula, his magic could be attuned to nature in a different way. He might be able to use magic related to the forest and spiders more easily. His magic could have a more primal feel to it, as he learned from a creature that is not part of the wizarding human society. He could also have a natural ability to sense danger in the forest, which could be an extension of his magical abilities.
Hermione would be a likely helper. She's smart and practical, and she could help Harry figure out what to do with all the stuff in the vaults, like how to properly inventory and distribute the items.
Definitely Harry. He's the central character of the original series, so many fanfictions revolve around him. However, Sirius Black is also extremely popular. His cool and rebellious nature makes him an interesting character to write about. People like to explore his relationship with Harry more deeply or create stories where he didn't die. And Luna Lovegood is a popular choice too. Her unique view of the world allows for some really creative fanfiction, like her discovering new magical creatures or having a special role in a new magical mystery.
Sakura might be affected. She could be initially shocked by Naruto's new look and then gradually come to see it as a sign of his growth and independence. It might even make her re - evaluate her feelings for him.
His family, if he has any remaining family members at this point, would also be deeply affected. They would be worried sick about his whereabouts and what could have happened to him. And any young witches and wizards who looked up to Harry as a hero would be left confused and disheartened by his sudden disappearance.
His closest friends like Ron and Hermione would probably be the most affected. They are used to having Harry around and relying on him. His sudden ignoring would leave them confused and worried.
Dumbledore would also be affected. As a mentor - like figure to Harry, he has certain expectations and plans for Harry. If Harry stops caring, Dumbledore's grand schemes against Voldemort might be derailed. He would have to find a new way to motivate Harry or adjust his strategies.
Harry might encounter some magical nomads. These are wizards and witches who don't settle in one place but travel around the wizarding world. They could introduce Harry to different cultures within the wizarding community and show him some unique magic that he has never seen before.
His friends like Hermione and Ron would surely be his first allies. They have been through so much together and would support his vision.