Guardian Angel

Her guise is that of a blue-skinned female angel with long white hair, white bird-like wings, and a poofy white dress. Her monster form on the other hand is mostly the same, but with an evil old woman/zombie-like face.

Unlike most depictions of angels, she doesn't have a halo.
Interestingly enough, there are indeed angels that are known for being malevolent. Somehow, she knew Finn's name without ever meeting him. Possibly implying her to be psychic. Jake was also tricked and captured by her, explaining why he was in the cage before she captured Finn.
She bears a strong resemblance to the Zoroastrian concept of guardian angels, in appearance and purpose.

Angel for short