The first hint that Celeste wasn’t all that she seemed to be came from her name, in the very first story!

Celeste: Celestial, heavenly. 

Further hints were found in her character profile for anyone who looked close enough:

She has no quantifiable age, created before humanity was born as the angel Saraqael. Though her host, Celeste, is a human, the one controlling her actions was never mortal.

The gender of Celeste is purely dependent on her chosen host. As a multidimensional wavelength of celestial intent, gender is meaningless.

In past millenia, she has taken male forms and so Saraqael is often associated with a male angel, though this could be no further from the truth. If given enough time in a host, however, she does tend to take on the mannerisms of that gender, like most angels.

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Saraqael : Prince of God. The Fallen Watcher. 

One of the seven archangels of heaven originally trusted to lead the heavenly host. One could call her current vessel an ironic twist, as she lusted for the daughters of men. Her primordial powers of protection have been twisted and corrupted, and she seeks to free her fallen brother by any means necessary.

This desire to break Lucifer of his cage is what brought her to Azazel’s notice originally, and he brought her under his control, an aspect of their relationship that chafed at her, though she put the demons he tasked to her to good use.

With Azazel out of the way, she can expand her influence without fear of retribution from the remaining Princes of Hell.

She has taken the time to learn true witchcraft since falling, discovering that it was an excellent substitute for her own celestial powers when she needed to avoid the notice of the angels tasked to seek her out and bring her to justice.

Those who are most favored fall the hardest.

–Castiel