My god. I go out drinking to celebrate the end of exams and everything happens around me.
So, Jinn's Charisma you pretty much hit head on with bubbling with barely contained elemental energy. It's pretty obvious she is riding the wave of her power, rather than controlling it, which can make her seem quite scary. She's also become quite adept at lying about who she is over the years, given she often wakes up as a different person, and most people don't buy that 'just happens' to her, so she needs to come up with plausible explanations as to how she got there.
As regards Jinn's changes: she only ever changes overnight, never during the day, and her changes are not voluntary. In short, she has no real control over her 'changeling' abilities - they are physical manifestations of the arcane energy of the Elemental Chaos flowing through her. She views herself as human, and she was born human - it was being vested with such power by the primordial that left her changing physically. She does end up in some forms more often than others, so perhaps she is developing some sort of control, but this isn't the ability to change her appearance at will, its the ability to 'pick' from a set group of appearances (namely the descriptions for each number). Her appearances seem more weighted to the Fire and Lightning descriptions, and very rarely is she seen as anything in the 6-10 range. (Mechanically, I'll be rolling to determine her appearance at the end of every short rest, and picking from Cold, Thunder, Fire, Lightning if it ends up in the 6-10 range).