how/ why does he take on such different forms? from an angel to a demon and vice versa?

He was an angel cast down by Archangel Raphael. In the end, there is no light without darkness, and darkness may not exist without light. Hens is portraying a delightful metaphor for Humankind. One may carry more predominant traits than the others, but we are all homeostatic balance, or imbalance, if you will.

To be more accurate, "homeostatic plasticity" is the consensus of the APA, amongst others, in modern times.
Historically, you can trace these normalised anomalies back to the beginning of written/portrayed/spoken history.