Ruko is Yuuto's older sister. She works as a private secretary to a company president. She is skilled in karate and is a 2-dan. She is a hard worker, though she drinks a lot at home and is unable to do any housework which makes Yuuto in charge of all the chores. She generally has a rough attitude and will often speak in a brutish manner. Since her parents are absent so often, Ruko takes up the position as the highest person in the house with her brother. When first meeting Haruka, Ruko was very pleased with her, though Yuuto regarded this as unusual.

(Source: Wikipedia)