Sonken Chuubou

Information

Introduced as the younger sister of Hakufu in the Great Guardians season. Hakufu was previously unaware of her existence. She appears to be very knowledgeable, as she was able to quote legendary strategist Sun Tzu, (a bit of an irony since the Sun family was said to have descended from Sun Tzu) a great contrast to Hakufu whom has trouble reading. It is implied that she might have feelings for "Koukin Onii-sama."

wikipedia

*Later it was revealed that her true name was Shoukyou, who was suppose to be Koukin's wife in history.