"Ah, the suffering... the sweet... suffering." -Bael upon being introduced in the series.
Bael is a ruling King in Hell, holding a staggering 66 legions under his command plus a full set of seven Cardinals. He is revered as, possibly, the most powerful Demon of his time, usurping the throne of God of Hell from Lucifer as well as overthrowing the seven principle lords of Hell, sending the traditional political system out the window and creating an anarchical chess board, a total of 72 "rulers" of Hell vying constantly for power and waging a never ending war against each other for territory and title.
Bael, having stolen the throne from Lucifer himself, has taken possession of the mythical Qliphoth Key from Ashtaroth, whom he has also claimed as a "bride" of sorts. The Qliphoth Key is an item that grants him access to the Qliphoth, the source of evil in humans. Despite this great power, he has no way of venturing deeper into humans' Sephiroths, thus limiting his capability to mere temptation, deception, etc. However, with enough sin energy generated, Bael is capable of transporting his minions to Earth in physical manifestations rather than through dreams or thoughts.
He is the principle antagonist of Season 2, being mentioned only briefly in the first season. He plans to reinforce his armies with the souls of all humans on earth, thus being able to fully bring every army in Hell under his wing and launch a massive attack on Heaven, making him effectively the Ultimate God.
Bael's appearance varied drastically. What could be called his released or true form is a black liquid with red, pupil-less eyes all around it (sometimes, these eyes will have slitted pupils for a menacing look). This liquid is entirely shaped by Bael's will or mind, taking the forms of humans or even inanimate objects, such as guns or swords.
To this end, Bael is a master shapeshifter. He can replicate almost anyone's exact appearance down to the last detail, including their voice. However, the most trouble he has with these replications is mannerisms. The Qliphoth section of the mind doesn't include simple habits like a twitch of the eye or biting fingers when stressed. An attentive, detailed person can quickly point out these faults and identify the replication as an imposter.
Bael's most favoured human appearance is that of a pale-skinned man with black hair that hangs in strands around his head, each strand having a white eye design on it. His eyes are red with white slits and a black pupil at the center. He often wears red, white, and black in these appearances, his trademark colours. Bael, in this form, has a differing voice than the one in his true form. In his true form, his voice is described to be a mixture between deep and soft, raspy and elegant, mature and childish. In his human form, his voice is cool and soft yet unnerving, with a hushed whisper or a choke. At times, it is suave and sophisticated and in others it is sarcastic and demeaning.
"Agonizing, isn't it? I know... I will enjoy making you suffer. And I will especially enjoy making you enjoy it."
Bael is, arguably, the most morally sick individual the series has to offer. He has no rules or restrictions, nothing keeping him from doing anything he can to break a person. Using her Qliphoth, he was able to take the form of Tiffany's father and make her relive her days of abuse back at her trailer, complete with raping her in front of a corpse of her mother. This results in a mental jarring that never escapes her, giving her a fear of being touched and a perpetual twitch in her eye.
Another example of Bael's cruelty is when he captured Sadow, Pandora, and Tanya as well as Reggie and Penny, and repeatedly decapitated Tanya in front of the four, bringing her back to life so that she can re-live the pain. When Sadow attempts to intervene, Bael rips the Hell-Hunter's spine straight out of his back but keeps him from dying so that he can remain stunned, in unbearable pain, and be able to do nothing as his friend is tortured in front of him.
Upon succeeding in capturing the souls of everyone on Earth, Bael begins syphoning the sin energy from every last one of them. The way he does so is by igniting their unabsolved sins, forcing them to go through a seemingly endless wave of agony as their minds re-live every sin they've ever committed, an act of such atrocity that brings a sort of feeling of ecstasy to him.
Even his own subordinates are either fearful or disgusted by him. Cain, a man notorious for his cruelty and savage nature, even gives a look of repulsion when he sees Bael having his legions rape pregnant women, deforming their children into mindless, Demonic tools. Witnessing this leaves Uriel in a coma-like state after an anxiety attack brought on by the actions she witnessed. When she tries to attack Bael, later on, the Devil-God retorts by burning off her wings and opening a portal to a realm filled with his soldiers, staring at her in lust.
As evidenced by the above, no other villain has surpassed Bael's magnitude of outright evil. During his personal arrival to New York, every step Bael took turned the ground black and ruined. Angelic beings felt a striking pain in their chest when near him, and felt an inability to move when he looked directly at them.