Hermione would likely be a main character. She might be really confused as to why Harry is suddenly acting less intelligent.
Snape could be another important character. He already has a low opinion of Harry in the original story, but in this fanfic, Harry hiding his intelligence might make Snape even more suspicious. He might think Harry is up to some new mischief or trying to deceive everyone for a malicious purpose. And Harry would have to be extra careful around Snape to keep up his act.
Maybe Harry hides his intelligence to avoid unwanted attention at Hogwarts. He could be tired of being singled out or feared because of his smartness.
Hermione would probably be one of the main characters. As Harry's friend, she could be a teacher at Hogwarts, perhaps the new Transfiguration teacher. She would offer her intelligence and knowledge to support Harry in running the school.
Maybe Dumbledore would also be involved. He would be the wise old wizard observing Harry's ice magic. He might see great potential in it and guide Harry on how to use it for the greater good, or he could be secretly worried that this new and powerful magic might attract unwanted attention from the dark forces.
There could be the mob boss who takes Harry under his wing. This boss would be a key figure, teaching Harry the ropes and acting as a sort of father figure. Maybe there's also a rival mob member who becomes an antagonist in Harry's story.
Neville could play a role too. He has a deep connection to the magical creatures, and he might be sympathetic to Harry's idea of hatching the basilisk. He could offer some advice on how to handle the hatchling based on his own experiences with plants and other less - dangerous magical beings. Also, he could be the one to stand up for Harry when others start to question his motives.
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.
Viktor Krum could be a major character. Since he is from Durmstrang, he might play a role like a mentor or a rival to Harry. Another character could be the headmaster of Durmstrang. He would be important as he sets the rules and tone for the school.
Another character could be a cologne - making elf. In the wizarding world, there could be elves who specialize in making colognes. This elf could have a special bond with Harry. It could be that the elf was tasked with making a special cologne for Harry's protection from the moment he was born. And as Harry grows, the elf helps him understand the true power of the colognes he has been surrounded with all his life.
Hermione could be one. She might follow Harry to America to continue their adventures together. She could help Harry adjust to the new place with her intelligence and knowledge.
There could be a young and brave goblin warrior who is eager to prove himself in the fight alongside Harry. And perhaps there's a goblin shaman or mage who has special powers and uses them to help in the battles. These goblin characters would each contribute in different ways to the story.