Yui Takamura

A First Lieutenant in Japan's Imperial Royal Guard, and member of a distinguished samurai family which has served the Japanese Shogunate for generations. As the daughter of a legendary soldier involved in the development of the Tactical Surface Fighter she has a strict character. Dispatched to Yukon Base as the Imperial Japanese attendant of the XFJ Plan. She and Yuuya don't see eye to eye, though surprisingly she does display acts of jealousy in ep.7 when she saw Yuuya hugging Cryska and mistakenly thought that the two were having a tryst, but actually Cryska collapsed and Yuuya simply caught her. It suggests that even if she and Yuuya seemingly are at odds with each other, she actually does care for him. Yuuya often calls her "Japanese Doll" usually behind her back.

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It is later revealed in the novel that she and Yuuya are actually half-siblings, sharing Takamura Masatada as their father.

(Source: Wikipedia)