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.
If we consider the common growth pattern in the Pokemon series, Charmander evolving into Charmeleon could be an early evolution in the fanfiction. Since Charmander has a clear evolution line and Ash has a strong bond with it, it would make sense for it to be one of the first to evolve, perhaps due to a tough battle or a special encounter that accelerates its growth.
One idea could be that Ash discovers a Sylveon in a forest. The Sylveon is injured and Ash takes it upon himself to nurse it back to health. This could lead to a strong bond between them, and Sylveon could become a unique addition to his Pokémon team, with new adventures and battles ensuing.
One reason could be that he doesn't want to stand out too much. In the fanfiction world, just like in the original Harry Potter story, being overly intelligent might draw unwanted attention, especially from the bad guys. He might be afraid of being targeted more if his intelligence is known.
Obviously, Ash is one of the main characters. He's the protagonist who has a passion for pokemon and is always eager to become a pokemon master. And the legendary pokemon he encounters or befriends would also be important characters. For instance, if it's Lugia, Lugia would play a major role in the story as it's a very powerful and mysterious pokemon.
His Pokemon might be a bit confused at first. Pikachu, for example, is used to their adventures on the road. But they'll soon adapt. They could make new Pokemon friends at the school too. And they'll be there to support Ash during his school challenges, like in battles or tests.
In fanfiction, Ash might improve his pokemon's IV through special training regimens. For example, he could focus on specific battle scenarios that target areas related to IV stats like speed or strength. He might also seek out rare items in the fanfiction world that have the power to enhance these inherent values.
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.
I'm not entirely sure as I haven't read that specific fanfiction. But generally, it might be about Ash's adventures with his Pokémon in a rather unique or different way compared to the original Pokémon story. It could involve new challenges, relationships, or character developments for Ash and his Pokémon.
Obviously, the Legendary Pokemon would be a main character. It could be a Pokemon like Mewtwo, who is often depicted as having a complex personality. It might start off as a mysterious and powerful entity that Ash has to befriend or battle against.