Lucifer
Lucifer is the first of the Fallen and formerly the leader of the Heavenly Host.
The precise nature of Lucifer's fall is not known to mortals. The Host claim that he was banished for trying to defy the Creator. Lucifer insists he was defending individual freedom and choice. Among confidants, he's even insisted that his rebellion is a false flag that is part of the Creator's plan. What's known is that he took a sizeable portion of Heaven's harmony with him when he fell, fundamentally altering the balance of power on a cosmic scale. Those Fallen who supported Lucifer retained much of their angelic powers as well.
Lucifer manifests as a male of the given species he's associating with, usually with the physical archetypes associated with members of the Host. For humans, he prefers a handsome, virile male figure with dove-white wings. However he has been known to take on female form at times, confusing inadequate human definitions of gendering a gender-less creature.
Lucifer amuses himself by nudging mortals this way and that, seeing who yields to temptation or breaks under duress. He is forbidden from directly interfering with mortal events, though like his brothers Gabriel and Uriel, he's found loopholes in his contract to exploit.
Lucifer currently is the de facto ruler of Hell. After several millon years of taking out his frustrations on unrepentant souls, he's grown somewhat bored with the job and largely leaves the day to day work up to a Triumvirate of demon lords.