Hmm, I'm not sure if that was actually a true Demon Lord. The first Demon Lord, Bael showed awareness and was actively looking for Solomon, but this one didn't seem to be aware. It didn't mention its name and didn't seem to be looking for Solomon. So I'm pretty sure it wasn't one of the 72. Plus the Demon Lords were suppose to be Homunculi, it should have had a form beyond mist. It could be one of the smaller ones like Malphas or Camio I guess.
As for Angie being one of them, that could be the case. Maybe her desire for Solomon was replaced with her desire for Belgrieve? It would help to explain her obsession, but that doesn't explain why she had a human form when she was first found.