Of course to show Raul who he was dealing with, none of them except for Rose and Vera really know what the devil looks like. It's why Raul felt so guilty when he told Rose that he gave them up to the devil. Had he known who he was beforehand, he probably would've said or did something else to prevent his brother's death without revealing their plans.
shareAs much as I'm loving the show it seems that they're going with a "realistic" approach as much as possible, with angels and demons being represented by human beings, I doubt we'll ever get any cool-looking "monster"-ish demons, so if for an episode the only supernatural-ish thing we physically see happen is glowing eyes, so be it I guess. At least he doesn't flash them at every appearance. And I got the distinct impression he was using them to intimidate Raul.
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"Betty's voice brought darkness to the land." - Amanda Tanen