Ekaterina "Katja" Kurae Initially taken in by Miyuki as an innocent exchange student, Ekaterina is actually a copper-manipulating Qwaser associated with Athos, though she doesn't always show complete loyalty.
She is capable of faking complete naivety and kindness very well, an appearance that she always displays with Miyuki and any non-dangerous person.
She cast Hana as her Maria from the start, subjecting her to constant BDSM and humiliation, but developing a deep affection for her.

(NSFW)