Mio Sanada
The school nurse and inventor of the G-On technology wielded by Yuuki, Sera, and Yayoi. She has the largest breasts of the entire female cast. Her true identity is that of Mako's younger sister (her real name is simply Sana). At some point in the past, she fell through a time warp and was aged several million years, causing her to go from the body of a 10-year old into a 20-something. She built Zero and created both the Maiden and Man Circuits. When she first came to Earth, her spaceship hit Ichiro and she transformed him into a cyborg, implanting the Man Circuit within him.