When I saw that Mokou was upright relative to Keine, I assumed dutch_angle. Then I saw that her hair falls "down." The remaining possibility is that Mokou is standing at an odd angle, but Mokou's hair is not obeying the same gravity as Keine's.
I'm inclined to say that's an error and the discrepancy didn't occur to the artist when he made the picture. Still nice work, wouldn't have noticed that part if someone hadn't pointed it out.
