Helium

Helium is an unusually thoughtful, quiet child (especially around strangers), usually more comfortable around adults than her peers. If she's unfamiliar with you, you can maybe get a few monosyllables out of her if you try, but she can grow to trust people and reveal herself little by little after a while. A dreamer, she has her own little world which she vastly prefers to the real one, and you'll find her aimlessly floating somewhere, head in the clouds and nose in a book, more often than not. She's always thinking about something and loves to read up on difficult concepts, often things you wouldn't expect for her age. When she does warm up to you, she's indeed a very sweet, sunny girl, if you don't mind a bit of precociousness and some attitude and smart-alec remarks from time to time.
She's unable to walk on her own but doesn't use a wheelchair, instead she's devised a clever floating alternative using balloons tied to a seat when she's hanging around outside, and indoors she'll usually hold herself up with just one or two balloons. It looks kind of impractical at first glance but she's actually very good at controlling these improvised mobility devices, not least due to her control over her element, and there are few places she can't get to.