Fleeting Shadow is a runner, runners move messages from generals to commanders and so on, as well as getting supplies or ponies from one place to another safely.

Every army depends on its runners to deliver where magic cannot, some armies use pegasi since they can fly, others use earth ponies who are more resistant to poison and stronger, others use unicorns for their magic.

Celestia has a runner who cannot be seen.

Fleeting was cursed, he cannot be seen except by ponies who he has physically touched.
He lives a life unnoticed and invisible, in service to Celestia.