Kokage Kuga (空閑 木陰)

A dark-haired girl of diminutive stature. Kuga's perpetually expressionless face and bizarre behavior suggest a morose, perhaps sinister character, but rather than depressed, withdrawn, or morbid, she is in fact playful, mischievous, and jocular (and occasionally morbid). The author suggests that her eccentricity is actually an affectation, designed to help her communicate with others. Asō and Kuga sometimes ride the train together in the manga.

(Source: Wikipedia)