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.

Proven Guilty
Lasciel is implanted in Dresden's mind when he when he picked up her coin at the end of Death Masks to prevent it being touched by the infant Harry Carpenter.

She can manifest herself to Dresden as an illusion that requires some concentration to see through. She often portrays herself as a tall athletic blonde woman in a white Greek-style tunic reaching down to her knees. She often tempts him with various abilities, which Dresden generally resists, but eventually enters in a bargain, stating he needs her help (typically in the form of her photographic memory) if they both were to survive.

As time progresses, Dresden eventually begins to accept more of the shadow's help, allowing him to remember the entire text of The Word of Kemmler by simply glancing at each page or helping him to understand obscure languages ranging from Ghoul to Ancient Etruscan in the hall of White Court. She also simply augmented some of Harry's spells with Hellfire.

White Night
In White Night, 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. Throughout the rest of that book, Lash's appearance becomes progressively more disheveled. By the end of the book, she does indeed rebel against her originator, apparently sacrificing herself in a bid to save Harry from a crushing psychic attack by Vittorio Malvora.

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