Leviathan

Leviathan, Prince of Jealousy, Prince of Hell. Father of Lillin. A member of House Corvin, member of Fionn's Squad.
A gruff, melancholic, jealous gritty sonofabitch. Drinks and plays piano to soothe his pain, when he's not fighting. A brilliant tactician, general and sergeant. A firm believer in judging someone by their actions and not by their words.
First appeared in Season 2.
Pre-Appearance
A close friend of Lucifer he was the first of the angels to join The Morningstar in his rebellion against Yaweh. It was due, in part, to his brilliant tactics that their rebellion enjoyed many successes. When a lull occured in the fighting he grew suspicious and warned Lucifer of a trap- advice that his friend rejected. Leviathan's instincts were correct. Lucifer took the bait that Yaweh had laid and he, and his rebels, were rounded up and cast out of Heaven.
Upon being cast down into Hell, and the consequent chaos, Leviathan was identified as a natural leader amongst the Denizens of Hell and was encouraged to become the Prince of Envy. He stepped forward, alongside many of the other commanders of the rebellion, and ruled the Circle of Envy for hundreds of years. 
From a distance he fell in love with the succubus Lilith. Believing that he would never stand a chance of love with her he fell into a deep envy of the affect she had on other demons. Seeing the confidence she had with Lucifer he came to be convinced that they were lovers- and it tested his friendship with his comrade.
Season 2
A close friend of Lucifer who felt very betrayed when Lucifer left Hell to go to Earth with Morrigan.
He was Lucifer's general in the War in Heaven, a master tactician.
He got together with Fionn Corvin when Fionn was trying to stage the Rebellion In Hell, helped him overthrow Satan and killed Asmodeus in revenge for the murder of Lilith.
During the events of The Tartarus War he discovered that his Jealousy aspect had hidden from him the most precious thing in his life: during a brief encounter with Lilith he had fathered the child Lillin. In that moment his aspect almost killed him, Fionn saved his life by forcing him to be calm and to embrace a new aspect.
Leviathan accepted Fionn's offer to become House Corvin's God of Sergeants.
Season 3
Season 4