Mei (メイ, Mei) is a member of the Cyclops, the original inhabitants of the O-Parts Hunter Planet. At the beginning, she had a huge crush on Jio, and saw girls Jio talked to as adversaries. As a little extra, she actually considered Cross to be a rival as well. She acknowledged that the matured Cross is beautiful, female almost. Surprisingly, she and Ruby become fast friends, despite Mei feeling envious of her beauty and Jio's feelings toward her. She apparently met Jio during the time-skip when Jio was nursed back to health by her village. She joins the party with Jio (by sneaking onto Orphan) after the time-skip, and is now traveling with the party. Mei's older brother Tsubame, the former leader of the Cyclops village, disappeared around the time Jio entered the Cyclops village.
During the fight with Tsubame she unlocks the power of her third eye and with Ball's help is able to defeat her brother. After Ball fights with the changed Tsubame, Mei realizes that she used Jio as a replacement for losing her brother, and now shows affections and love for Ball. It is recently revealed that Mei's father was Kirin and Kujaku's cousin, making Kirin and Kujaku her second cousins. She enjoys sweets, and found the mayor of her village, her grandfather, extremely annoying.
Inside the Zenom base she, Kirin, and Jajamaru fought against Kujaku. Using her third eye she was able to take control of Kirin's katana and helped him deliver the final blow. She was later a witness to the potential Armageddon that nearly consumed the world and one of the last to hear Jio Freed's voice before he disappeared into the void for now. Years later she now has twins and has settled with Ball. Her face is never showed during this scene but her hair and outfit are clear give aways she tells her children the story of their youth and then ushers them to bed putting down the book next down to the Tricky O-part.
After the battle is finally over,  it is revealed the Mei and Ball eventually get married and had two children together, a son, Jio, and a daughter, Ruby. Their son has Mei's hair and third eye, and the daughter has Ball's black hair and Mei's regular eyes, either twin has the potential to become an OPT but from their genetic inheritance and other past examples it's most like Jio will not be an OPT and Ruby will. They also wear a scarf around both of their necks that looks exactly like Jio Freed's. Much like Kirin and Kujaku, they seem to have different attributes from both parents.