(from ebookjapan):

Even though it is a mock drill, Karen Saeki is not just a smart, tall, good-looking man who can defeat the criminal landscape with a single blow... but she is a full-fledged female police officer.

She uses her keen sense of observation and manly energy to hunt down the culprit. The next case is that of her twin brother, Tsubasa Shinozaki, a popular idol, who was found dead in Tokyo Bay. Trouble between close blood relatives, grudges, jealousy, and all manner of other problems lead Karen to the full extent of the case.