The Necromancer is an undead demon that was sent by the Gods in the Halls of Judgment to aid Sethra Lavode and Her Majesty, Zerika the Fourth reestablish the Dragaeran Empire as well as defend Dragaera from the Jenoine. She chose to remain with Sethra thereafter.
The Necromancer comes from a place called the World of Seven Doors, and it is said that she "knows six of them...and she is not entirely unacquainted with the seventh," a fact that even astonishes several of the gods.
She appeared to have no name when she first manifested on Dragaera (or at least none that she cared to use in that plane), so she accepted the respectful title of "The Necromancer", given to her in admiration by Sethra the Younger, in honor of her unprecedented powerful facility with planes of existence and necromancy in general. (Earlier it is shown that the goddesses Verra and Moranthë are also extremely impressed with these abilities.)
Her personality seems distant and very reserved. She has few social graces and tends to take every question absolutely literally. Most of her social failures are because she is unfamiliar with Dragaeran etiquette. When she first manifested on Dragaera, she didn't realize it was impolite to invite herself into Dzur Mountain by force and send Tukko sprawling. Luckily, she was slightly more gifted in conversation and did not irk Sethra further.
She has on at least one occasion made a big show of the gestures and incantations to close a portal, though it's not known if they were necessary. Verra admits that she herself isn't as fast at these things as The Necromancer is, so perhaps a little showing off is justified.
She has a certain artistic side, and Vlad claims she painted a large work depicting a dragon defending her young. (However this work is also attributed to Kathana e'Marish'Chala making it unclear who the real painter is) She is also generous, and has gifted Sethra Lavode and Zerika with similar garments -- Sethra's being black, Zerika's being gold -- that each include a jeweled necklace and a jeweled hip-loop.
According to Vlad, The Necromancer is much more comfortable dealing with you if you are dead. This is likely why she gets along so well with Sethra Lavode. Vlad finds her creepy, though she seems to like him. Vlad apparently helped save her soul once by parlaying with Eastern ancestors in Morrolan's window tower.
Among other planes that The Necromancer is comfortable with, she can appear to one who is Dreamwalking and hold normal conversations. She has also, at various times, reanimated armies of the dead, and opened and closed portals between the planes as a matter of course.
Her current place of residence is a very cramped quarters in Castle Black, the lack of space alleviated by the fact she is free to move in dimensions other than the three familiar ones.
On Death: "It is the limitation of one's ability to reach certain phases of reality... It is sometimes inconvenient."