Answer:
You saw me where I never was and where I could not be. And yet within that very place, my face you often see.
Lighter than what I am made of, More of me is hidden Than is seen.
When liquid splashes me, none seeps through. When I am moved a lot, liquid I spew. When I am hit, color I change. And color, I come in quite a range. What I cover is very complex, and I am very easy to flex.
When set loose I fly away. Never so cursed as when I go astray.
What is it something that you always have but you always leave behind?
Special abilities and brightly colored underwear are all you need to be one of these.