A boy unable to feel anything, be it happy or sad. To fit in with society, he pretends to react how others normally would. After he tells his doctor, Kisaragi, about his predicament. Later, Kisaragi reports Kuu to the Earth Eradication Army, saying he's who they're looking for.

Spoiler

Kuu's family and anyone who had any connection to him at all were killed by the army to sever those connections from him. When this happens, he joins the army because he has nothing better to do.