Lasciel

Lasciel is a fallen angel, and a member of the Order of the Blackened Denarius. Lasciel is also known as the Seducer, the Webweaver and the Temptress.

Lash
Lash is a shadow of the fallen angel Lasciel, which exists in Harry Dresden's mind. She came into his mind when Dresden picked up Lasciel's coin at the end of Death Masks to prevent it being touched by the infant Harry Carpenter.

She was offered to Harry Dresden during the events of Dead Beat. A copy of her has taken up residence in the unused portions of Harry's brain, with the intent of tempting him into taking up the coin and becoming a member of the Order of the Blackened Denarius. Harry can converse with it and has taken to calling it "Lash".

Lash can manifest herself to Harry, becoming an illusion that can take 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.

Lash often tempts Harry with various abilities, which he refuses, but some of the time he bargains with her, saying that she needs to help him (typically in the form of her photographic memory) if they both are to survive.

As time progressed, Harry eventually began to accept Lash's help, allowing him to remember the entire text of the Word of Kemmler by simply glancing at each page or helping Harry to understand languages ranging from Ghoul to Ancient Etruscan in the hall of White Court. She also simply augmented some of Harry's spells with Hellfire. The pet name influences Lasciel's shadow to aid Harry, and give itself up to save Harry's life.

It was stated by the author that the story of Lasciel and Lash was not yet over, hinting at a reemerging future importance of the characters.

Lash's presence may be hinted at in the end of Ghost Story. According to Uriel, Harry was cheated out of free choice by a Fallen who, in addition to having millenia of experience with human behaviour, knew Harry and his foibles well. However, this could be attributed to any number of Fallen, including those not among the Order of the Blackened Denarius.

Demonreach attributed Harry's survival to three beings: Mab, who kept Harry breathing, "HERE" (i.e. Demonreach), which kept him nourished, and "the parasite", which kept Harry's blood flowing. Bob's last mention of Lash in White Night was that the shadow of Lasciel had to use some of Harry's soul in order to take on free will and become Lash.

The name Lasciel may derive from the Latin, "lascivus", which can mean wanton, playful, frisky, lustful, lewd, etc.

Fan speculation
Due to one of Lasciels titles being the Webweaver, it is possible that Lasciel's Denarian form is that of a spider.