All throughout the series, specifically, within the second Arc of the story, there are references to Subaru smelling like the Witch's Scent, Giving us the knowledge that he is Definitely connected to her.
Why is this? It's well known that Subaru was pulled from modern day Japan into this Fantasy world, as well as that he has the ability "Return to Death" In which he has the ability to Respawn.
These things most likely have things in common with his relations to the witch, though... What exactly is the relationship? How are they connected?