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.
The main heroine and the only girl who doesn't wear glasses at the very start. She possesses abnormal strength and is the last of the G-On Riders to join. In the first episode, she loses her panties (which have strawberries on them) in a monster attack and they fall into the hands of Ichiro, who struggles to return them to her. Eventually he does return them. However, Yuuki losing her panties is a running gag in the series. She also on occasion loses all her clothing, such as during the final battle.
Nurse Sanada's older sister, even though Mako is physically younger due to Sanada having fallen into a time warp. She communicates with Riro through a pair of bobbly antennas on her head, which Riro later uses to mentally control her. Yuuki saves her from this mind control with the help of Sanada, Pao, Ai, and Zero. She is the leader of the aliens and also pilots the alien's pink UFO.
A green haired alien who is the first to realize that humans are good and that Earth is a nice place to live. She eventually quits trying to attack earth and befriends Yayoi.
The alien behind the attacks on Earth. She is physically the youngest of all the aliens. Over a hundred thousand years ago, Riro came to Earth and gave her glasses to a young primitive girl. These glasses were the catalyst that lead to the formation of human civilization. Riro's glasses eventually ended up in the possession of Yuuki. Without her glasses, Riro's heart became cold and she lost all emotions save for the desire to destroy. She is redeemed once Yuuki returns her glasses. Riro and Yuuki form a close bond by the end of the series.
Han's father and a renowned doctor. Han says that his father doesn't really care about him much and rarely bothers with him. A scandal involving an illegal abortion that he performed on a minor, which resulted in death, eventually arose, ending in him losing his job. Because of the infamy that it brought to his family, he and Han constantly moved from city to city.
Sumire is a shy blonde girl who meets Azusa, Ui and Jun after Yui and the others graduate. She is a freshmen, two years below the other light music club members. She is currently working in the Kotobuki household as a maid and originally came to the light music room to try to retrieve Tsumugi's teaset that was left behind after she graduated. After being scared off a few times by Sawako, she eventually agrees to join the light music club and eventually becomes its drummer. Similar to Tsumugi, she is quite talented at preparing tea, but feels inclined to keep her role as a maid a secret from the others due to the fear of being punished by Tsumugi. As her family, who came from Austria were taken in by the Kotobuki household before she was born, Sumire grew up as Tsumugi's playmate, often regarding her as her older sister and inadvertently sparking Tsumugi's interest in yuri by introducing her to manga of that genre. As she grew up and learned the truth about the relation between the two families, Sumire felt she needed to regard Tsumugi with more respect while in public. Sawako reveals that Tsumugi asked that the tea sets be left in the club room. However, before the beginning of the new term Tsumugi had asked Sumire to remove the tea sets thereby ensuring that Sumire would encounter the other members of the light music club and would be invited to join. Sumire's situation is revealed to the surprise of the other members of the club by Sawako. (Source: Wikipedia)