Sid and Sadie Smithfield-Wyatt were Shades twins who "died" in an occultist ritual in 1977. Their deaths appeared to be a double suicide. Jane Quaint took over their house and continued their work after their demise.
Appearance
Sid and Sadie are said to wear mostly the same clothes and look outrageously identical, even though they are fraternal twins. They wear odd clothing and makeup, and speak in a posh British dialect. The two of them "died" in their early twenties, so it is safe to assume that they appear that way in the novels.
Background
Little is known about Sid and Sadie's background. It is said they grew up in a wealthy family until their parents died in a car crash, from which they survived and gained the Sight. The twins inherited their family's money at a young age and traveled the world in search of a way to "defeat death." They later bought a house in which they recruited Jane and others to help them do so. A ritual was performed by them and their followers in 1977, which kept them both in a death-like coma until The Shadow Cabinet.
The Shadow Cabinet
Sadie's first act in the novel is to call Stephen and Rory to come upstairs to the library. The four ask each other basic questions, while Sid and Sadie try to figure out what makes Rory so special, and successfully guess that Stephen's a member of the Shadow Cabinet.
Sid and Sadie follow Rory and Stephen after they escape the house, but are in no hurry to catch up. It's unknown if this was because Sid and Sadie had absolute control inside the dimension or not.
Sid and Sadie are the last to leave the dimension, and revive right after Rory, Sid first and Sadie next. Sadie tries again to figure out what's so special about Rory. Jane tells her that Rory's a living terminus device, and adds that she (Jane) has given Charlotte the Sight, and wants to asend to be with them. In response, Sid cuts Jane's throat with a nearby knife. He cradles her as she dies and coaxs her to let go, then terminates her ghost when she tries to come back. They explain to Rory that although they cared for her, Jane wasn't part of the elite like they were, on top of conferring the Sight to a non-seer, so once she had served their purpose, she had to go. Suddenly, Stephen jumps up and holds the remaining knife to Sadie's neck which she pulls away bare-handed. It doesn't hurt her at all, and the wounds health quickly. Sid and Sadie fight all the good guys off, although Rory pours the rest of the drink from Jane's ceremony down Sadie's throat, knocking her unconscious. Sid advises everyone to stay there, since the police are coming, and that they should put the stone back in a safer place before ghosts overrun London. He tells Rory he looks forward to seeing her again, before dragging Sadie out.
On December 21st, Sid and Sadie walk into Hardwell's Bookstore and casually kill both their ally Clover the shopkeeper,and his assistant. They're happy to discover that Clover collected everything they need, and leave the shop.
Appearance
Sadie is described as willowy, fairy-like young woman with "feathery blonde hair." Her eyebrows are so pale they appear invisible, and she is dressed in a long white gown. Egg blue eye shadow covers her eyes, and she also has a crescent moon drawn on her forehead.
Personality
Sadie is shown to be clever and quick, in both humor and intelligence. She seems to be the smarter of the twins and is harder to distract than Sid. Sadie also has no fear of death or pain, as shown when she easily pried a knife from her neck with her fingers, slicing them in the process. She is kind and friendly to everyone, a skill she uses to manipulate people into cooperating with her. Murder is not beneath her either: she has nonchalantly discussed and performed mass murder on acquaintances (by poison).
Sidney "Sid"
Appearance
Sid is tall, blonde, and wears a fedora. He appears nearly identical to his sister, though they are only fraternal twins. He also wears odd blue makeup.
Personality
Sid is the more playful twin; he has flirtatiously teased both Rory and Stephen. He is quick-humored and smart like his sister, though Sadie's focused demeanor could peg her as the brains of the bunch. Sid had also assisted in a mass poisoning, slit Jane's throat, and (possibly) staged the car crash that killed his parents. Both twins posses superhuman abilities.
Restoration
After the siblings' ostensible suicide, their bodies remained intact for decades while they lingered somewhere between life and death. In The Shadow Cabinet, Jane Quaint forces Rory to take part in a ritual that allowed Sadie and Sid to return to the realm of the living.
SPECTULATION ABOUT WHAT THEY ARE NOW:
The most likely case is that Sid and Sadie have Transcendent Ghost Psychology, and now have god-like ghostly abilities. Although they are not immortal, some of their powers may include: Astral Manipulation, Astral Plane Manipulation, Teleportation Manipulation and Portal Manipulation. However, this is just speculation; the only power used by Sadie in cannon so far is Ultimate Intangibility.
As for the specific type of ghosts they're supposed to be, Sid and Sadie have loose similarities to Revenants. According to Wikipedia:
"A Revenant is a visible ghost who has returned from the dead to terrorize the living...Those who revive from the dead are typically described as wicked, and vain wrongdoers in their lives, whose bodies didn't decay after death."
The article goes on to say that "...In Finnish ghost stories, Revenants are usually visually indistinguishable from the living, and are usually mistaken for the living at first. They are often well-dressed, up to being out of place."
They also share similarities with the Strigoi, since their true supernatural nature is ambiguous; the Strigoi is mostly connected to vampirism, but also to troubled spirits, and living people with magical properties.