Lucy Carlyle
Lucy Carlyle is the teenage first-person narrator of the story. She is the only female agent working for Lockwood and Co. In that capacity, she is a "Listener" because she is the only who can communicate with ghosts. Meaning, for the time being, she can only hear them.
What she is not expecting is to hear the skull in the jar. It did speak once, but that was seven months ago. Since then, nothing, but suddenly she can hear it speaking again and the things it says are snarky and disturbingly close to things she has thought herself.
This sudden talkativeness sets the stage for the biggest change Lucy will undergo. Not only is she able to hear the skull, but without warning she is able to engage in two-way communication. This is a historical event of the highest type. A human communicating with a Type-3 ghost is almost unprecedented.
The Whispering Skull
The title character has also been known simply as the skull in the jar. But that was before he suddenly became so loquacious. The skull has taken his time to begin whispering, but once he starts, he almost won't shut up. His ability to communicate makes him a Type-3 ghost.
Lucy definitely would not mind him being less talkative. The skull is sarcastic, mean-spirited, and occasionally psychotic. He is prone to dispensing advice that is not even close to anything that the agents of Lockwood and Co. are willing to do.
The skull also turns out to be useful, however. He feeds Lucy important information that does prove useful in carrying out their investigations. The skull ultimately saves Lucy's life, which certainly scores him some points. Unfortunately, Lucy is still convinced the skull is evil.
Albert Joplin and George Cubbins
Albert Joplin is introduced as a "tiresome little scholar" but this underestimation will prove nearly tragic. He actually an associate of the head of Sweet Dreams Excavation and Clearance. An encounter with a ghost named Bickerstaff becomes the moment in which things begin to change.
Things really begin to go off the rails when Joplin seems to establish an unexpected bond with George Cubbins, another member of the Lockwood and Co. agency. Both George and Joplin have looked into a bone-glass. It is a mirror capable of causing anyone who looks into to die while shrieking. Both Joplin and George have begun acting strange since that moment.
Eventually, it is revealed that Joplin has been impacted by the bone-glass to a far more dangerous degree than George. While George is able to be saved through the intervention of his friends, Albert Joplin is not as lucky. In fact, Albert Joplin has gone quite mad.
Anthony Lockwood
Anthony Lockwood heads up the agency which bears his name. The action essentially kicks off—and is driven by—the moment when Lockwood issues a very significant and very serious challenge. The challenge is tendered to Quill Kipps and his very successful and much better funded Fittes Company.
Some of the whispers made by the skull to Lucy are warnings. Specifically, they are warnings to her that Lockwood is not to be entirely trusted. Lockwood has kept secrets and so that skull's warnings are not the first time these suspicions have been raised. Especially on the subject of what lies behind the door of the room on the second floor which Lockwood has forbidden anyone to enter.
The challenge to Kipps heats up when both agencies are assigned to investigate the mysterious disappearance of the bone-glass mirror. Lockwood's familiarity with a licorice-loving relic-girl named Flo Bones increases his odds of winning the challenge when she points them in the direction of Julius Winkman and a highly anticipated—and top secret—auction of haunted relics.
