The Jitter Girls are sisters and a group of villains that appear in Death Bringer. Not much is known about them besides the fact that they give Valkyrie and Skulduggery a rather hard time, and they probably would have killed them both if one of them had not caused Valkyrie to change into Darquesse. It is indicated that they are incredibly powerful, as even China Sorrows expressed her surprise when she was informed that Skulduggery and Valkyrie had managed to defeat them. Derek, the author of the series, has confirmed that there are only three Jitter Girls.
"Any poor soul tortured like that isn’t going to rest easy. They have too much pain to deal with by themselves, so they need to spread it around. That’s what I think, anyway. The truth is nobody knows why they came back, or why they started killing people. But that’s what happened."
The Jitter Girls are six year old Triplets possessed by a dark being at a young age in the 1930's. The dark being was said to have tried to sneak into this world through their minds, planting a "seed" in them. It is said to have probably not been a Faceless One . They were thought to be possessed by the devil and Skulduggery was called in, but couldn't help. They died in an asylum. However, they returned to life and now live in a box.