Be a Ninja
only if no ones there to see it (a doppleganger is not a ghost its just someone who looks excactly like another person, but not related)
You can
In the Aeneid: Book I, Venus conjures a cloud to shroud Aeneas and his friend, Achates, so that they can enter the city of Carthage without being seen.
Blend in with your enviorment and act normally
Flexibility is being able to move your arms and legs without pain or tightness.
Probably.
Be quiet. Moose get disturbed easily.
so that it can hunt its prey without being seen
Lions can move. It'd be quite difficult for a meat-eating, predatory animal to survive without being able to move.
According to Bernoulli's laws of motion no
Not if you are under 18.