Often, the plot develops by showing how Harry tries to maintain his normal routine despite being sick. He'll go to classes, participate in magical duels (although perhaps not as well as he usually does), and still try to be involved in any mystery or adventure that's going on. The tension builds as the people around him get closer to finding out the truth. There could be a moment where he almost faints during a crucial event but still manages to hide the real reason behind it.
Well, it might start with Harry subtly avoiding certain situations where his sickness could be revealed. For example, he might skip Quidditch practice, making up excuses like he has too much homework. Then, as the story progresses, his friends might start to notice little things, like he looks paler or he's more tired than usual. But Harry keeps coming up with believable lies to cover it up.
One reason could be that he doesn't want his friends to worry. Harry often takes on a lot of responsibility and might see his sickness as a burden he doesn't want to place on others. Another reason could be related to his enemies. He might fear that if Voldemort or his followers find out he's sick, they'll see it as a weakness and target him more aggressively.
He might pretend to not understand complex concepts. For example, when others are discussing advanced magic theories, he could act confused even though he actually gets it.
He may also choose to stay quiet during discussions where his knowledge could shine through. Instead of speaking up and showing off his intelligence, he lets others do the talking and only chimes in when it's necessary or when he can play dumb and ask simple questions.
In some fanfictions, Harry might hide his money in a secret compartment of his trunk. He could use magic to create a hidden space where no one else can find it.
He could use magic items like the invisibility cloak all the time. In fanfiction, the cloak might be enhanced to not only make him invisible but also mask his magical signature, so no one can detect his presence through spells.
In many fanfictions, Naruto might hide his pain by putting on a big smile. He's always been the type to try and be positive in the face of his friends. He could also distract others with his goofy antics, like making a joke or doing something silly to shift the focus away from his inner turmoil.
Harry might initially be confused. He has always looked up to Dumbledore, so when he realizes Dumbledore is trying to control him, he could start to question everything he thought he knew about their relationship. He may become more rebellious as a result, trying to break free from Dumbledore's influence and make his own decisions.
In fanfiction, Harry might use his knowledge of the secret passages in Hogwarts to hide from Dumbledore. He could also use Polyjuice Potion to change his appearance and blend in with other students or people in the wizarding world. Sometimes, he could even ask his friends to create distractions so that he can slip away and hide. For instance, Ron and Hermione could start a commotion in one part of the castle while Harry hides in a different, more secluded area.
Occlumency helps Harry hide abuse in fanfiction by allowing him to create mental barriers. These barriers prevent others from accessing his thoughts and memories related to the abuse. For example, if a wizard tries to read his mind to find out his weaknesses, Harry can use Occlumency to block out the memories of the abuse at the Dursleys.
One possible reason could be for protection. Maybe there are threats out there targeting him, so by hiding his appearance, he can move around more freely without being recognized. For example, if there are rogue ninja who want to take revenge on him for past battles, hiding his face would make it difficult for them to track him down.