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.

Description
She seems to be a black sheep among the other Fallen; "a rebel angel among rebel angels." As such, she isn't trusted by Nicodemus and her coin isn't given to people lightly.

Forthill describes her as "The Seducer", and she's also called the Webweaver and the Temptress. It seems odd to Forthill that Nicodemus would want to free her (by giving her to Harry) since she does not follow Anduriel's lead.

Details

 * The sigil on her coin is a "vaguely suggestive of an hourglass."
 * Her coin was supposed to be in secure storage in a Chilean monastery. Yet, Nicodemus had it to toss at little Harry Carpenter.

Death Masks
At the end of Death Masks, Nicodemus tossed the coin in front of toddler Harry Carpenter just as Harry Dresden was walking up to the baby. Harry's subconscious decided to  pick up the coin instead of picking up Harry himself. Harry buried it in his lab within his circle under concrete.

Blood Rites, Dead Beat, Proven Guilty, and White Night
Afterward, in books Blood Rites, Dead Beat, Proven Guilty, and White Night, a copy of Lasciel took up residence in the unused portions of Harry's brain, with the intent of tempting him into taking up the coin permanently and becoming a member of the Denarians. Harry could converse with it.

Dead Beat
In Dead Beat, at Bock Ordered Books, Harry encountered "Shiela" who seems to have perfect recall and helps Harry find Die Lied der Erlking. She called Harry saying she's scared so he met her at Bock's. That night, after he used Hellfire Harry had an odd dream with a hot tub set in a cave. Lasciel appeared to him to make him an offer to aid him in fighting the Necromancers by teaching him secrets of his craft unknown to mortals. Harry saw through her offer of Power and declined.

Later, as Shiela, she recites the entire Erlking book for Harry by memory at her apartment. Afterward, they kiss. Harry returned to Shiela's place to check on her concerned for her safety. He discovered that Shiela was an illusion that Lasciel created for Harry wanting to help him against the Heirs of Kemmler fearing that the death of Harry would mean the death of part of her.

White Night
In White Night, Harry taken to calling it "Lash". Lash is considered something of a separate character from the "real" Lasciel at this point in the series.

Harry talks about temptation and choice with Lash. He suggests that maybe being inside of him has had some affect on her, just as she has had some effect on him. When Lash is taken back into the whole, she will be gone: die. He then plants the seed that Lash has free will, too and that she can choose for herself, have a life outside of Lasciel.

In the battle of the Deeps, during Vittorio Malvora's crippling attack of despair, when Harry and his friends about to be overrun by ghouls, and a bomb is about to go off, Lasciel speeds up his mind to effectively slow down time. She makes a bid for Harry to call the coin to him—he refuses. Lash concedes she has a choice. Then, she tells Harry that Vittorio Malvora is possessed by an Outsider. Due to the circumstances of Harry's birth, he has the potential to wield power over Outsiders. Harry can shake off the spell and she will give him the opportunity to do so. The she walks away toward Vittorio.

Lasciel's coin was given to Father Forthill in White Night, who placed it with the rest of the coins the Knights of the Cross had collected. As far as the readers know, the coin is still there.

Skin Game
In Skin Game, Lasciel comes back briefly using Hannah Ascher as the host. She is motivated to kill Dresden by her desire to save her child - the parasite. However, after Hannah's death, her coin is stuck in Hades' vault.