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.
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.
It could potentially fall into the mystery genre if the 'hide' aspect involves some sort of mystery, like Harry Styles hiding something or being hidden. For example, it could be about a hidden identity or a secret he's keeping.
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 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.
Maybe he's been injured in a battle against dark wizards. In the wizarding world, battles are dangerous, and getting cursed or physically hurt can cause great pain, leading to him crying.