Encountered: Chapter Seven - at the library, mentioned in Chapter Eight - in Seth's train.

After Aislinn is attacked in the park and saved by Donia, Donia takes her to the library to clean up in the bathroom:

'Then she stepped back into the seemingly empty hallway, letting the bathroom door close softly behind her.
Donia stood, invisible now, talking to one of the bone-girls. Like the rest of the bone-girls, this one was ghastly white and so thin that each of her bones could be seen under her almost-translucent skin. The fact that she was mobile seemed to break some basic law. Surely things that looked so frail should have trouble moving? But the bone-girls glided over the ground without any visible effort. Despite their cadaverous mien, they were beautiful to watch.
[...]
"Find them. Find out why they attacked the girl. If anyone compelled them to do so, I want to know. Aislinn is not to be harmed."
The bone-girl's voice was a dry whisper, as if the words had to rush over something rough before they found form. "Should I tell Keenan?"
Donia didn't answer, but her eyes darkened to the same oil-black sheen they had in Comix.
The bone-girl stepped back, hands held up in supplication.
[...]
Aislinn's voice wasn't much louder than the bone-girl's had been when she answered. "Sure. I'm fine."' (pg75-pg76)

Aislinn briefly mentions the bone-girl again in Chapter Eight, once they're in the safety of Seth's steel walls:

'So she told him. She told him about the guys outside the library, about Donia's rescue and fury afterwards when she walked to the bone-girl. She let the words tumble out, not holding anything back.' (pg82)

It is unknown if they are kin to the Scrimshaw Sisters, or if the bone-girls (and this one in particular) are encountered again during Wicked Lovely.