Yumeko’s background is largely a mystery. Her only known living relative is an older sister said to be hospitalized after a mental breakdown, for whom Yumeko pays hospital bills. As a Jabami, she is a distant relative of the Momobami clan.
Yumeko transfers to the elite Hyakkaou Academy, where gambling, deceit and manipulation decide one’s social status, and immediately gains attention from her fellow students for her beautiful looks. However, she draws the attention of the powerful student council and its president, Kirari Momobami, as she threatens to turn the social order of the school on its head by demonstrating her gambling prowess in successive victories against some of the school’s best gamblers.
Befriending Ryota Suzui, a student driven into slavery as a “house pet” after incurring huge debts to Mary Saotome, Yumeko accepts a challenge from Mary and shocks the Flower Class by defeating her in a game of Vote-Rock-Paper-Scissors even though Mary had rigged the game in her favor. Mary’s loss to Yumeko reduces her to house pet status, while Yumeko, who is indifferent to money and only gambles for the thrill of it, uses her money to pay off Ryota’s debt and free him from servitude.
Yumeko expresses her desire to challenge Kirari, the school’s best gambler, and makes a name for herself by facing and defeating several members of the student council. She does eventually face Kirari in a game of tarot cards, where the loser would voluntarily leave the Academy, which ends in a draw.
Some of Yumeko’s opponents, like Mary and Itsuki Sumeragi, eventually become her friends. Yumeko generally does not hold grudges or harbor ill feelings towards others, with the notable exception of student council member Midari Ikishima, a suicidal masochist whom Yumeko heavily dislikes due to Midari’s habit of rigging deadly games against herself to maximize the risk of self-harm. As Yumeko explains it, risk is part of the thrill of gambling, and Midari’s actions amount to hoarding all the risk for herself and denying it to her opponents.
Although Yumeko is a generally open-handed and cheerful person, she has demonstrated a manipulative streak, and often chases her compulsion to gamble recklessly to the fear and detriment of her friends and companions. When Rei Batsubami angrily asks Yumeko if she feels any remorse for the people whose lives may have been ruined because of her gambling addiction, Yumeko confesses that the only guilt she feels is over her lack of guilt.