Mimic...Eldritch...squints
But yeah, perhaps dark scleras, red eyes and some slimy grabby tendrils would be as far as I go for a doppelganger Theo. But the fun part....the fun part. HOW should he nom the real Theo? Grabbing hold of Theo with its tendrils and pull him in to itself and slowly absorbing him bit by bit~ Or maybe its entire front side bursts out trying to envelope Theo, quickly scrunching into a tinier form inside the doppelganger to digest him~