The Elder Sister-like One (NaN)

Since the death of his parents, young Yuu has suffered considerably from neglect and abuse at the hands of his adoptive relatives, who leave him to his own resources to survive. Fortunately, a girl named Chiyo comes to his aid and offers to take care of Yuu while acting as his older sister. However, Chiyo is not what she seems, and Yuu knows all too well that her intentions are far from innocent. In exchange for her generosity, Yuu must supply Chiyo with his "life potential."