Mika Nogizaka (乃木坂 美夏)

Mika is Haruka's younger sister in her second year of junior high school. Her hobbies include playing the violin, and playing squash. She has a personality that is exactly opposite her sister, who is naïve. She enjoys Yuuto's company and she often calls him "onii-san." When Haruka is with Yuuto, however, Mika likes to tease them. In contrast to her airhead older sister, Mika is very stable. Even at her young age, she is more knowledgeable about sex than Haruka, although she will get shocked or stunned mentally if exposed to H-rated material just like her sister. She is deathly afraid of ghosts.

(Source: Wikipedia)