Hermione would probably be a bit worried at first. She might think it's a waste of time compared to studying magic. But then she'd see the benefits of physical fitness and start to support him. Ron, on the other hand, would probably make fun of him a little but also be secretly impressed.
His other friends from Hogwarts might not even believe it at first. They might think it's some kind of elaborate prank. But once they realized it was real, they might try to find a magical way to communicate with him in Skyrim, or even try to open a portal to bring him back.
Hermione would probably be worried at first but then start researching all she can about space magic. Ron would be a bit more skeptical but also excited for Harry. They might try to find a way to communicate with him while he's in space, using some sort of enchanted device.
He might use magic to enhance his workouts. For example, he could use a spell to make the weights lighter when he's starting out and gradually make them heavier as he gets stronger. It would be really cool to see him combine his wizarding skills with gym exercises.
They would probably be shocked at first. I mean, Hogwarts reading fanfiction? It's a strange concept. But then they might be really curious about what Hogwarts is reading. They could try to get their hands on the same fanfiction to see what all the fuss is about.
Flitwick might be quite intrigued. He loves magic in all its forms and fanfiction would show him new and different ways magic could be used in the Harry Potter world. He'd be interested in the creative spells and concepts that the fans come up with.
One possibility is that Harry's late arrival at Hogwarts could change the dynamics of the relationships he forms. For instance, instead of being in the same boat as the first - years from the start, he might end up befriending some older students who take him under their wing. Also, his late start could mean that he discovers parts of Hogwarts that others haven't yet, like secret passages or rooms that are only accessible to those who arrive late or are in a certain situation.
He might be very sad at first. Harry has always valued his friends, so being forgotten by them would be a huge blow.
Perhaps Mal would make friends with a Hufflepuff. This Hufflepuff could be really kind and inclusive, and they would accept Mal for who he is right from the start. They could introduce Mal to the Hufflepuff common room and all the cozy traditions there. Another possible friend could be a Slytherin who has a similar background to Mal and they understand each other's motives and ambitions in the magical world.
Ron might see it as another adventure that Harry is having and would be eager to join in, even if he doesn't fully understand it at first. He'd be excited to see the different Pokemon and might even try to catch one with Harry. Hermione would approach it more analytically. She'd question how the Pokemon world operates, what the rules of Pokemon training are, and how it could potentially affect the wizarding world. She might be a bit worried about any potential negative impacts.
The characters' reactions would be diverse. Snape would likely be very critical and use it as another reason to dislike Harry. He might see it as a sign of Harry's lack of respect for the wizarding world's traditions. The other students would be a mix of those who are fascinated by this strange Muggle object and those who are worried about its potential for harm. Hagrid would be confused, not really understanding what a gun is but knowing it doesn't seem right in Hogwarts. Overall, it would create a stir among the Hogwarts community and lead to a lot of discussions and speculations.