Yukino Yukinoshita

Yukino is the leader of the Service Club, of which she is the sole member until Hachiman joins her. She is smart, beautiful, and is nicknamed the "Ice-cold Beauty" for her icy demeanor. She has a twisted sense of "noblesse oblige", and believes that it is the duty of the exceptional (in other words, herself) to help the "lost lambs" of the world. Her personality often leads her to be disgusted at the naivety of others, particularly Yui's, and she does not hesitate to show it. Despite this, Yui still affectionately calls her "Yukinon".

Source: Yahari Wiki