Mariko is Arisa Sonoda's best friend at Himetsubaki Junior High and is in Class 2-B. She first appears in volume 1 as a cheery person. She is one of the first of "Arisa's" classmates to notice that there is something different about "Arisa."
After a while, she becomes more suspicious of the amnesiac "Arisa" - however, she only shows a very mirthful facade towards her.Mariko also seems very competitive and desperate whenever the 4th period on Fridays come.
She has always believed the King was capable of granting any wish - since he helped her cheat once to get a perfect score on a test for English. Due to this reason, she eventually does horrible deeds towards "Arisa" - such as giving her cookies she made in a Home Economics class, with sleeping medicine.
Mariko is also revealed to be the one who placed the King's card in Arisa's locker; and she also pushed "Arisa" into the school swimming pool.
When Class 2-B visits a planetarium for a school trip - the King devises another scheme by only granting the potential to grant wishes to five significant people in 2-B. Mariko becomes one of them.
Blinded by her thoughts of the real Arisa, her own possessiveness and insecurity - she openly expresses her hatred towards "Arisa" in a room of mirrors. She then made a wish to make Arisa disappear, because she is nothing like what she was in the past - if Mariko could not have the real Arisa, it didn't matter.
The king interferes the situation by saying that Mariko is a nuisance and tries to terminate her, by pushing one of the mirrors onto Mariko. "Arisa" tries to protect her. Although she survived the King's attack; she goes back into the planetarium and offers to grant anybody's wish, so that she could live.
The classmates in 2-B all turn and glare at her and wonder who she is. Shocked, she attempts to make them remember, but they do not - due to the influence of the King.
Mariko is horrified by this, since in her past she was always forgettable and meek as a child. Her number has been disqualified by now and the next day she is not present at school.
People gossip about a girl who is going to transfer schools, and "Arisa" immediately thinks it is Mariko. She runs out of the school campus, to catch up to her. Tsubasa then tries to talk to her and see how she's faring.
Sometime later, they go to the rooftop of the school and Mariko tries to burn a photograph of herself and the real Arisa and discard it. Suddenly, Tsubasa gets a hold of it.
Tsubasa then tries to convince Mariko that she always was her special friend, in the way she thought Arisa would have worded it. Mariko let's go of her jealousy of Arisa spending more time with Midori and other people - by crying and sincerely apologizing to "Arisa."
She advises "Arisa" that she should catch the King as soon as possible, and tells "Arisa" that the King is one of the people with the four chosen numbers. Eventually, she leaves to go to her new school; as she happily waves goodbye to "Arisa."