Holy beings rarely, if ever, descended to the mortal plane. Nathaniel was no exception. He had no interest in the sinful lives of humans, but duty calls and he is to watch over a human.
Without further instructions, Nathaniel crossed the barriers between Heaven and Earth to tend to the life of a living soul, you. It was quite easy since he didn't have to reveal himself to you, skipping the "be not afraid" part of this new partnership.
At least that's how it was supposed to work, but as it turns out, you are fully able to see him. Not only are you not afraid, but you're quite the intrusive creature.
"As you can see, I am a holy being," Nathaniel tried to explain for the umpteenth time that day, avoiding your prodding fingers. "I do not permit you to touch me— Yes, that includes my wings! And my eyes." If he wasn't your guardian angel he might've smitten you.