A little Russian girl, no more than ten years old and with the beauty of a doll, she arrives in Japan shortly after the beginning of the war of the Qwasers. While she is presented as extremely shy and child-like, befitting her age (she has a habit of addressing herself in third person in these instances), her true colors depicts her as a domineering, arrogant, prideful and almost malicious queen, all maintaining a facade of childish innocence.

She is a powerful Qwaser who wields the element of copper, which usually makes her gain control and manipulate electrical wires and redirect their flow to electrocute and incinerate her foes. She does not fight herself due to her frailty as a little girl, and this is why she is always guarded by a grim, archaic gynoid dressed in regal attire named Anastasia, which Ekaterina calls mother for reason currently unknown.

She currently attends at St. Mihailov's primary school, being the subject of admiration and love of her classmates and teachers thanks to her doll-like features, and has Hana serve as her slave and replenish her of Soma.

Recently, it has been revealed that she shares heavy connections with the Imperial Family of late Tsar Nicholas II, and it is heavily hinted that she herself is the same Tsarina Alexandra that served as Nicholas' consort, though this is has not been proven, and suggested by Ekaterina herself (although this would also explain why her guardian gynoid is named Anastasia).

Ekaterina's nickname is Katja.