The theory is Mika is the sister of Arisu because they look similar, plus there have been several official art pictures which show them together in close proximity. It's thought that she is like Miho, in that she abandoned the Shimada Style of tankery and absconded to Keizoku High School, and was therefore disowned by the Shimada family, which is why her "younger sister" Arisu had to take up responsibility for being the next head of the Shimada Style instead. The above picture is a representation of that, in that her face has been scrubbed out of the image.
