Maybe he has some dark secrets in his past that he doesn't want to be known. For example, he might have been involved in something illegal or unethical that could damage his reputation in the world of the web novel.
It can create suspense. Readers will be curious about what he is hiding and keep reading to find out. For example, it might be revealed gradually through flashbacks or other characters' investigations, which adds an element of mystery to the story.
Maybe he is ashamed of it. Some past works might not be up to his current standards, or perhaps there are elements in the novel that he doesn't want to be associated with anymore.
The player might change the title and his name associated with the novel so that it becomes difficult to trace back to him. He could also use some legal means like copyright restrictions to limit its availability if possible.
Obviously, Harry Potter is the main character as it's about him hiding his power. His friends, like Hermione Granger and Ron Weasley, are also likely to be main characters. They might be in the dark about his hidden power at first, which could create some interesting dynamics in their relationships.
Perhaps he has hidden the full extent of his Sage Mode abilities. He might be able to stay in Sage Mode for a much longer time than he has shown so far, or have some other undisclosed powers within that mode, like the ability to sense enemies from much greater distances.
Sasuke could be one of the people who help him. Despite their past differences, they have a deep bond. Sasuke has his own skills and knowledge that could be useful in Naruto's situation, like his Sharingan abilities which could help in detecting enemies while Naruto is hiding his face.
Draco and Hermione is also a pairing that shows up in these types of fanfictions. Given their different backgrounds and the initial animosity between them in the original story, Draco hiding his pregnancy from Hermione can lead to a complex story. It could involve Hermione slowly finding out and having to deal with her own feelings about Draco and the situation. There's also the pairing of Draco with a random Muggle, which can add a whole new layer as he has to hide it not only from the wizarding world but also from the Muggle's family and society.
It could be a new character. Maybe a powerful ninja from a far - off land who has heard rumors of Naruto's strength and is determined to draw it out and defeat him to prove his own superiority. This antagonist might be secretly spying on Naruto and trying to find ways to force him to show his true power.
The Soul Reapers could be involved too. Some of them might notice that Ichigo is acting strange and try to figure out what's going on. Captains like Byakuya or Kenpachi could potentially cross paths with Ichigo during this time. And then there are the Hollows. They might sense that something is different about Ichigo but not be able to fully understand why he seems weaker than before, which could lead to some interesting confrontations.