Harry Dresden describes Helen as having "a calm face, entirely void of emotion. It was a little spooky. She had the look I'd seen in films, on the faces of prisoners released from the German stalags at the end of World War II. Empty. Numb. Dead, and just didn't know it yet." She claims that she's "a functional borderline psychopath... Heartless, calculating, empty, and can muster very little in the way of empathy for my fellow human beings." This is discredited somewhat by her deep mourning over the loss of her daughter, who took a bullet intended for Marcone, as well as the obvious affection she shows towards her fellow members of the Ordo Lebes.
She was married to Greg Beckitt, and they had a daughter named Amanda Beckitt. Amanda took a bullet meant for John Marcone from a jealous gang member, but was presumed to be fired by a Jamaican gang, three years before the events of Storm Front. The shooter was never found by the police, so they resolved to get vengeance upon Marcone, who they thought was responsible. He offered them reparation, but they turned it down. They attempted to file a wrongful death lawsuit, but couldn't win against Marcone's lawyers.
Since they couldn't beat him by legal means, the two eventually hooked up with Victor Sells, participating in rituals to help make the ThreeEye drug and death curses in order to sabotage Marcone's business. They were both later charged with drug distribution.
In the series
In Storm Front, Beckitt and her husband, Greg Beckitt, hire Linda Randall as a driver, which causes them to cross paths with Harry Dresden. After Linda is killed, Dresden learns that the Beckitts' daughter was killed in a cross fire between John Marcone and a Jamaican gang. The Beckitts attempted to lodge a wrongful-death suit against Marcone, but Marcone's lawyers were too good, getting the suit thrown out before going to court. Apparently, Marcone offered to pay them blood money to make reparations, but they turned him down.
Later, Victor Sells found the Beckitts because of their money. He offered them vengeance on Marcone in exchange for their help. When Dresden confronts Sells, the Beckitts are at his house, naked and involved in the ritual. Once interrupted, they both start shooting at Dresden. Once the hostilities are over, the Beckitts are arrested, for being naked among other reasons.
In White Night, Helen Beckitt resurfaces, as a member of the small magical group Ordo Lebes. She's suspected of helping the murderer of the practitioners, however Harry Dresden soon realizes she is innocent. Karrin Murphy looks into the Beckitt's history and states that Greg Beckitt was killed in a prison riot.
Helen Beckitt is now working for John Marcone, under the alias of Ms. Demeter - an amusing pun, but also a reference to the Greek goddess. She also seems to be Marcone's lover. We are not completely certain why this is, but it can be assumed that she is working and sleeping with a man she hates in order to gain his trust so that she can stab him in the back at a later date.
Later, Harry Dresden engages in a soulgaze with her, wherein the scars from the trauma of watching her child die are confirmed. Dresden witnessed her memory of watching her child murdered in gang violence perpetrated by Marco Vargassi who intended his bullet for John Marcone. The soulgaze implies that she's stuck in a loop, repeating the incident in her head over and over again.
In Small Favor, Harry Dresden visits her in her office at Executive Priority Health to get information that will help him find and rescue Marcone from his abductors. Ms. Demeter refuses at first. Then Torelli and four of his thugs barge their way in, demanding records in an attempt to take over the business while Marcone is held hostage. Dresden stops Torelli and gets him to leave with an assist from Thomas Raith. Ms Demeter gives Dresden an address that leads him to a suburban safe house where Hendricks and Gard are hidden.
Later, Dresden visits her office on the way out from seeing Marcone. He lets her know that he knows it was her that gave the Denarians the location of Marcone's panic room. He has no intention of ratting her out. Dresden might want information from time-to-time, no strings, either way, he won't say anything to Marcone. This time Torelli makes a good fall guy as the traitor.
The name Demeter
It was Marcone who suggested the name Demeter as a pseudonym for Helen Beckitt. It is based on the Greek Myth about Demeter whose daughter, Persephone, was abducted by the Lord of the Underworld and held in Hades and Demeter froze the world in her grief so that nothing could grow. In Death Masks, Marcone is secretly providing long-term care for Helen's daughter, Amanda Beckitt who is in a permanent coma for having taken a bullet intended for Marcone. So, Marcone is Hades, Helen is Demeter, and Amanda is Persephone.
Marcone told Harry Dresden that he can not return Persephone from Hades because Helen believes that Amanda is dead. In Marcone's words: "The child's death nearly destroyed Helen—and her world is still frozen. If she knew her daughter was... trapped... just lying there in a half-life... it would shatter her world. I shouldn't wish that."