Lash

Lash is the name Harry Dresden ultimately gives the shadow placed in his mind by the Fallen Angel Lasciel. It first appears in Death Masks.

Death Masks
In Death Masks, Lasciel was implanted in Harry Dresden's mind when he when he picked up her coin to prevent it from being touched by Harry Carpenter.

Dead Beat
In Death Beat, when Harry Dresden identifies her in a dream as Lasciel, she explains that she is a reflection of the true Fallen Angel residing within his mind.

Dresden first meets Lash as an assistant at Bock Ordered Books named Shiela. Shiela is very attractive to Dresden, and after helping him recall what was written in Die Lied der Erlking, they kiss. When he returns to her apartment with Waldo Butters, Dresden figures out that 'Shiela' was just an illusion in his mind, and really a manifestation of Lash. She can create illusions in his mind, as she demonstrates by making the building appear as if it is on fire. Dresden does, however, manage to exert control over her, as she resides in his brain.

In another dream sequence, Dresden and his unconscious double meet with Lash and come to an agreement that they will use her for information, but not rely on or trust her. Following this meeting, she teaches him how to block out the pain of Quintus Cassius' torture.

Proven Guilty
In Proven Guilty, Lash can manifest herself to Harry Dresden as an illusion that requires some concentration to see through. She often portrays herself as a tall and athletic blonde in a white Greek-style tunic reaching down to her knees. She often tempts him with various abilities, which Dresden generally resists, though he does make use of her intellect and illusions from time to time.

Dresden eventually begins to seek out more of the shadow's help, and when he is captured by Lucius Glau and Darby Crane, he asks her to show him how to break the faerie manacles. After doing so, she grants him the ability to see in the dark in order to escape the Full Moon Garage.

When examining the Carpenter family's panic room, Lash informs Dresden that the Ward protecting it is the work of an archangel, probably Rafael.

Lash again shows her power over Dresden's senses when he tires to use Little Chicago to find Molly Carpenter. She causes his burned hand to feel agonizing pain in an attempt to prevent him from doing so as she believes it will harm him and by extension, her. After he rejects her renewed offer to take up the coin, she leaves him alone.

Dresden eventually admits to Michael Carpenter, who he had been avoiding, that he picked up Lasciel's coin. Carpenter already knew, inquiring about Lash, who chuckles in the back of Dresden's mind.

White Night
In White Night, Harry Dresden gives the Shadow the pet name "Lash" explaining it as a bit of petty insolence towards a nigh immortal being, and pushes her towards considering herself as a separate entity from the original Fallen she came from.

Dresden seeks her help on a number of occasions. She shows him a mental reconstruction of Thomas Raith's apartment, and reveals to him that someone was concealing themselves at the Ordo Lebes meeting. She translates from ancient Sumerian a conversation that Dresden has with some ghouls after they attack Anastasia Luccio's Camp Kaboom. During the White Court gathering, she translates the ancient Etruscan the vampires speak.

With a little over 90 seconds before a bomb is due to detonate in the Raith estate deeps, Lash appears to freeze time by accelerating Dresden's mental processes. As he is likely about to die, she tries again to get him to take up the coin. When he refuses, she appears to him, looking quite disheveled and crying. She tells Dresden that Vittorio Malvora has been possessed by an Outsider. She hints that some confluence of circumstances surrounding his birth has given Dresden the potential to hold power over such Outsiders. Lash then apparently sacrifices herself in a bid to save Dresden from a crushing psychic attack by Malvora.

When Bob later examines Dresden's head, he explains that Lash had been using some of Dresden's soul to provide him with power, but that she has now been erased as a result of Malvora's attack.

Word of Jim
According to Jim Butcher Lash reappears in Ghost Story under a different name.