Misawa Natsuki♀

Eiji's classmate. Taken into Rumi's family by her father as a child, Natsuki was raised as his second natural daughter. However, her fierce devotion to Rumi has given her the appearance more of her maid than her sister. At a glance she's the inscrutable, mysterious type. She doesn't talk much and doesn't let her emotions show, but due to the development of her body at Rumi's hand when she enters "H" mode she undergoes a complete change, to a brazen, dirty talking girl. Because Rumi has focused on her anal training, Natsumi is still a virgin. She alternates between sadistic and masochistic personalities at Rumi's command. She is initially discontent with Eiji and his blossoming friendship with Rumi, viewing it as a rivalry; over the period of her acquaintance with him however, she grows fond of and closer to him herself. Given her openness to partners, Natsuki is also bisexual.

[From J-List and Wikipedia]