The main character Dominic has spirit magic, and is able to see them. Although they are incredibly rare, object spirits are a little more common. A simple remainder after an object has spent enough time with a soul. Tsukaiyin calls this "spirit echho" and is unable to percieve the visual of these little creatures. In cases like Todd: his spirit is actually anchored to the vessel: allowing him to remain in Chandra's dimension (Which of course travels).