She was part of the occult community of Chicago, and a young woman who Harry Dresden helped teach to control her magical talent. She was described as "a tall woman, buxom and lovely in an old-world way, with pale, pretty skin and round cheeks well used to smiling." She had long dark hair and dimples.
In the series
In Fool Moon, she approaches Harry Dresden at McAnally's, asking him to help her with a greater summoning circle, claiming it is for academic purposes. He refuses to give her the information, because he doesn't think that Delaney has enough knowledge or experience to be messing with it, and knows that the White Council wouldn't want the dangerous information in the hands of a non-Council member. Angry at Dresden for not telling her, Delaney storms out of the bar.
When Dresden shows up at Harley MacFinn's house on Chicago's Gold Coast, he finds Delaney dead. She was killed by Harley MacFinn when he transformed into the Loup-garou. She had been trying to recreate a circle that could hold in the werewolf, after his previous circle was ruined, but as she didn't know how to empower the circle properly, it had been unable to hold in the beast.
She seems to have been a kind and thoughtful person. She smiled often. She was an environmental activist who participated in a lot of fundraisers and protests (which is where she met MacFinn - a year before the events of Fool Moon, through the Northwest Passage Project). She offered to pay for Dresden's dinner, even though he wouldn't give her the information, because she knew that he was strapped for cash. She tried to help Harley MacFinn, even though she knew that she was going over her head and that there was a chance that what she was doing could get her killed.
Dresden feels incredibly guilty over her death, and very responsible for not giving her enough data to make an informed decision. There are many parallels between her situation and Susan Rodriguez's later on. The early mistake with Delaney and Rodriguez undoubtedly made it so that he now has a more honest and open relationship with his later apprentice, Molly Carpenter.
In White Night, Harry Dresden mentions Delaney when consoling Elaine Mallory over Anna Ash's death. Dresden felt responsible for her death, and notes that revenge didn't make him feel better. He visits Delaney's grave occasionally to leave flowers and remind himself no one wins them all.
Word of Jim
Have you killed off anyone you know in the books?