Well, Ron is also a candidate. He might get tired of all the chaos in the Gryffindor common room or just need some time away from his friends to think. 'Taking the air' would be a good way for him to do that. Luna Lovegood could also be a character who 'takes the air'. She has a unique perspective on things, and being outside in nature might be something she enjoys, perhaps to observe some magical creatures or just to be in her own world.
It could be Harry Potter himself. Since it's a Harry Potter fanfic, he is often a central character in many such stories. He might be leading his friends on some airborne adventure.
In Harry Potter fanfic, 'take the air' could potentially refer to going outside for some fresh air. It might be used in a context where a character decides to leave the stuffy interior of Hogwarts, like the castle or a classroom, and step outside into the grounds to get some air. This could be a moment of relaxation or a chance for a character to think privately.
In 'harry potter is betrayed fanfic', Luna Lovegood might be a character who betrays Harry. Maybe she is under the control of some mind - altering spell or has been deceived by a false prophecy that makes her think she is doing the right thing by betraying him. Also, Hagrid could be a shock betrayer if he is somehow forced to choose between Harry and his beloved magical creatures, and chooses the latter in a way that is seen as a betrayal to Harry.
Even Ginny could be written as a betrayer in a fanfic. For example, if she becomes possessed by a dark force, she might turn against Harry. Her relationship with Harry is so central in the original story that a betrayal from her would be a huge shock and create a lot of drama in the fanfic.
Harry Potter himself could be a main character. He has a history of standing up against injustice, so it's likely he'd be involved in a revolt. Hermione would also be a key figure. Her intelligence and knowledge of the wizarding world's laws would be invaluable in planning and leading a revolt. Ron, with his loyalty and courage, would be right there with them.
Harry Potter would probably be one of the main characters. He's the central figure in the original series, so it's likely he'd be involved in this fanfic in some capacity, perhaps as the hitman or someone related to the hitman - style plot.
Most likely, Harry Potter, Hermione Granger, and Ron Weasley will still be the main characters. After all, they are the central figures in the original series. But there could also be new characters introduced who are either part of their wealthy new circle or those who are affected by their millionaire status.
Obviously, Harry Potter himself would be one of the main characters. He's the hero of the original series and his story would likely continue in this fanfic.
Obviously, the illithid would be a main character. It could be a mysterious and powerful being that Harry encounters. Also, Hermione and Ron might still be important characters. They could be either suspicious of the illithid or try to help Harry understand it better. Dumbledore might also play a role, perhaps guiding Harry in his interactions with this strange new creature.
One interesting plot could be the characters discovering new flying spells or magical creatures while in the air. For example, they might encounter a flock of rare flying phoenixes that lead them to a hidden magical place.