Harry could downplay his academic achievements. If someone asks about his grades or how well he did in a certain study, he might say it was just by chance or that he got a lot of help. He may also avoid participating in intellectual discussions or competitions in the fanfiction world to keep his intelligence under wraps.
Another way is by making silly mistakes on purpose. When doing magic spells or solving problems that require intelligence, he could deliberately make small errors that make it seem like he's not as smart as he really is. This way, others won't suspect his true intellectual level.
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.
One reason could be that he wants to avoid unwanted attention. In the fanfiction world, being overly intelligent might make him a target. Another reason might be that he is trying to fit in with a certain group that values other traits more. For example, if he's in a group that values physical strength over intelligence, he may hide his smarts to seem more like them.
He might use simple language on purpose. For example, he could pretend to be confused about things he actually understands well. He may also make silly mistakes intentionally to give the impression that he's not as smart as he really is.
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.
Well, it could strengthen his friendships. His friends might respect him more for being open about his intelligence. For example, Hermione would be glad to have someone as smart as her to discuss complex magic with. Ron might initially be a bit jealous but would soon realize how useful Harry's intelligence can be in their adventures.
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.
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 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.
He might want to protect himself. If others know how smart he is, he could become a target. Enemies might see him as a greater threat and go after him more aggressively.