The answer is that her face is painted like a skull because she got her ass dragged off the Ninth to the First as part of a long con that required her to use a rapier instead of a broadsword, fake a decade of cavalier training she never got and dress like a goddamned nun. This is all lame and Gideon is highly aware of this, especially since there's another teenager within arm's length of her and that teen looks like she should be on the Ninth instead of Gideon.

"Because skulls are cool." Gideon answers instead, tipping her head to the side. "Why don't you paint your face?"