So this is based on the Hellboy franchise of 2004 - 2008, directed Guillermo Del Toro.
Alastor - Hellboy
Charlie - Liz Sherman
Angel Dust - Abe Sapien
Original character - Professor Trevor Bruttenholm
Sir Pentious - Rasputin
Pentious is a priest who once served God but the evil of mankind drove him to turn against the lord and begin a cult that worships the two overlords of Hell: Beelzebub and Lucifer. Solomon, Pentious's former friend, a fellow Christian priest, and exorcist tries to sway him back to the light but he refuses and vows to seek vengeance on God but destroying his greatest creation: Mankind.
Some years later Solomon hears word that Pentious's cult has kidnapped a woman called Abigail from Louisiana and he sets out with his fellow exorcists to recuse her. When he locates the cult he discovers that have found an ancient doorway which can unleash hell on earth but it can only be opened by two beings born of the common things that lure man to sin. One born of beauty and lust, the other born of fear and wrath. However these beings must have a connection to both hell and earth for this to work. Therefore the cult produces two children who carry both human and demon blood. The first one, the daughter of beautiful Lucifer and his willing human bride Lilith. The second one, the son of fearsome Beelzebub and Abigail the abducted human woman he raped.
That night the already born daughter and the pregnant Abigail (Who at the moment is in labor) are presented to the cult. But the ritual is interrupted by the exorcists who managed to rescue the woman, the baby girl, and arrest most of Pentious's followers. However Pentious himself escapes. Medical help is no where near and Abigail is suffering complications. She begs Solomon to save her baby no matter what the cost, not at all disgusted or horrified by the child despite how it was conceived and who fathered it. She dies giving birth to a son and Solomon (Despite orders to do so by the Vatican) cannot bring himself to kill the two infants who are rumored to bring about the end of the world.
So Solomon takes in the orphaned babies, naming them Charlotte and Alastor and decides to raise them to protect humanity instead of harming it. As he cares for them, he also opens up a secret bureau that captures demons or other supernatural creatures with the purpose of eliminating or reforming them. Charlotte "Charlie" and Alastor grow up together and are extremely close however as they get older the end up having different lives, views, and interests.
Charlie is beautiful and can easily pass for a regular human so she's allowed to be outside. She's ashamed of her demon half and is afraid of her demonic powers, embracing her human half and trying to fit in with everyone else but also firmly believing that in the reformation part of the bureau. Alastor on the other hand is more fearsome and can barely look human at all, so he has to be hidden from the outside world. He feels nothing but pride for his demon half, would use his demon abilities freely if not restricted, feels his human half weakens him, and believes more in the elimination part bureau. They are both highly trained agents who hunt demons and the supernatural.
Some of their living findings on the course of reformation are Vagatha "Vaggie" a demon who ran away from her demon family, Anthony "Angel Dust" a demon who was abandoned by his demon family, and Husk a demon who just doesn't want to be eliminated. Friendships form, feelings develop, and tensions rise.
But what really causes trouble is when Pentious returns to finish what he started years ago. Open the gate to hell by using Charlie's soul as the key and Alastor's body as the one who turns it in the lock. Solomon fears for Charlie's life and for what choices Alastor may make.