Nami

ナミ

A young girl who washes up on the shores near the location of the training camp. She looks to be about 10 to 12 years old. However, her name and age are all unknown. When she wakes up from after being rescued by Yasumi and Shouko, she is found unable to speak. However, despite this, she makes up for her inability to communicate verbally with animated facial expressions, giving her a cute, childlike appearance. She becomes particularly attached to Shouko and Yasumi. Her seiyuu is Rina Satou in the Aoi Shiro visual novel game and drama CDs. (Source: Wikipedia)

She is Yasumi's twin-sister who was turned to vessel of Yaobikuni's soul by their father. Her real name — Tsunami Nekata