I had to have it explained to me, as well. A Shadow is a human's suppressed thoughts and feelings, given physical form. They usually appear to be more negative and volatile, when encountered.

[ A sage nod as she pauses to inspect bright yellow vase that is, in all actuality, taller than her and close to Sebastian's chin in height. Wow, she could fit in this. ]

You said "masters"? Are you a demon servant, sir?