One of the new characters introduced in the CW Seed animated series Constantine: City of Demons is the comic book character Nightmare Nurse. I won't talk much about what the character does in the animated series other than how she makes her entrance, but if you want absolutely no spoilers, then move on to another story.
The image above is how she first appeared in Phantom Stranger Vol 4 #8, which credits her creation to Dan DiDio, J.M. DeMatteis, and Phillip Tan. This explains not only that she is in the series but the choice of her first words as DeMatteis is the writer of the animated series. And Asa, her real name, says the exact same line above about having been in the middle of a threesome with a Dryad and a Slyph. She also makes a disparaging comment about the quality of sex she had previously with John Constantine, which you can see from the panel below she did in the comic as well.
Constantine (Matt Ryan) and Asa (Laura Bailey) have similar banter in the episode, but the reason she was called is very different. In the comics she is called in after the Phantom Stranger is stabbed and mortally wounded by the Spear of Destiny, which was in the hands of the Question at the time. The members of Justice League Dark use their magic to try to save him, yet fail. This leads Zatanna to call in the Nightmare Nurse. She is compelled to help those in need having been forced to swear the Hippocratic Oath by Apollo and Panacea. The Phantom Stranger is without his soul after the attack by the Spear of Destiny, leading Nightmare Nurse and Zatanna to go and retrieve it from the land of Non.
She would next appear after the Trinity War, when she comes to heal Constantine, taking on the identity of Zatanna to help bring him to his sense in the House of Mystery. She teams up with him and Swamp Thing to battle Blight.
She has a boatload of powers, mostly magical-based, including Astral Projection, Healing, Illusion Casting, and Genetic Manipulation. She appears to be a weapons master too — which seems to go against the Hippocratic Oath.