Angel Leading the Israelites out of Egypt.

I guess I've read this a million times (well, at least every year when we study passover) and I never realized hat there was a literal angel leading them out of Egypt. Could they see it? What did it look like? Was it appointed to that job? What did it do later? Or before that? I've heard that there are angels appointed to the nations . . . .what's he been doing for 2000 years? I bet s/he's busy right now. . . .LOL