Mariko Takeda is the older daughter of Takeda, who discovered a parallel dimension and was sentenced into a magical slumber.
She later reawakened as a powerful vigilante called the Black Queen.
Long ago, Mariko was once a prominent scientist like her father. One day, she discovers a portal to another dimension. When she got there, she saw a world of vast technology and awesome magic, that dimension was called the Fast World.
But when she returned to Earth to tell her father about her discovery, Mariko suddenly felt ill and went into a deep sleep. Takeda seeks to destroy all magic, as he believes that it is responsible for his eldest daughter's illness.
Mariko, Takeda's daughter, accidentally discovered a different and magical world called the Fast World. Yet, unfortunately, something happened and her mind seems to be trapped there while her body is in a state of coma in the real world. Blinded by anger, Takeda placed her body in a water tank to keep her alive via liquid breathing, while using his freezing technologies in tracking down the Guardians and trying to kill them. He is assisted by a mysterious boy from the Fast World, named Liam, whom met Mariko and fell in love with her.
Liam acted as a spy on the W.I.T.C.H. girls for Takeda, infiltrating their magical school bus and planting evil and dangerous creatures from his world into Heatherfield in order to attack W.I.T.C.H. and putting their magical students in danger.
Mariko battled against the ruler of the dimension as the Black Queen, leading a group of the Fast World's natives called the Black Army, until W.I.T.C.H. were fooled by the White Queen, Arkaam, into attacking her, and so was imprisoned with Shinobu and William for a while. Her body floats in a tank full of liquid in Takeda's laboratory under stasis-lock.
In the Fast World she met Liam and the two of them fell in love. Mariko saw the evil intentions of the White Queen so she decided to leave her body on Earth and let her mind fight against Arkaam as the Black Queen.
In the end, Mariko suffers the loss of her lover Liam.