A Girl Named Yumeko Jabami is the very first chapter of Kakegurui written by Homura Kawamoto and illustrated by Toru Naomura.
The story begins with Ryota losing to his classmate Mary Saotome and thus becoming a pet were he is seen being used as a slave and being physically abused by schoolmates and being used as a bench. After Yumeko Jabami enrolled on the same school as Ryota and Mary, Ryota shows her around the school. Later on, Yumeko is being challenged by Mary for a "Rock-Papers-Scissors" gamble were Yumeko loses all her chips during the gamble, but the gamble continue's when Yumeko is making a huge risk of a bet with 10 million yen.
The story begins when a boy raises 10 chips, which 1 chip is 10,000 yen. Having faith with the cards he has in his hands, he exposes them revealing them to be "full house". His opponent smiles viciously revealing a "royal flush" making the boy losing the poker game. The girl tells the boy that he has a hard time, and to continue hard the next time finishing the line with "dog".
Later on, the girl wins in a game of poker. She calls out the boy who lost a match with her, asking him to bring her melon bread and red tea. The ones who are around her are asking the same thing to the boy. Later, the boy is seen beaten by other students, and even taking his wallet were he tells them that he has no money. Once the boy returns, the girl calls him telling him that her legs are feeling numbs and uses his back as a bench.
Later, a transfer student is attending the school. She introduces herself as Yumeko Jabami to the rest of the class. The teacher asks the students that it would be nice if someone could guide her around the school. The teacher picks the boy who's name is revealed to Ryota Suzui to show Yumeko the school. Yumeko is been shown around the school and is impressed on how the school looks like. Then, Ryota tells Yumeko if she knows about gambling. There, Ryota explains the situation of the school and asks her to be careful if she gets invited to participate. The laughs sinisterly, saying that it is interesting.
Once the schoolday is over, Yumeko is surrounded by her male classmates. This stirs the girl who have beaten Ryota that is heading towards Yumeko. She introduces herself to Yumeko which her name is revealed to be Mary Saotome. She asks Yumeko if she has time asking her if she wants to play gamble with her. This peaked interests to her classmates.
Yumeko is asking her what game they are about to play. Mary tells Yumeko to play a game called "Vote Rock-Paper-Scissors" saying that it is an original game. This peaks interest in Yumeko as she agrees to it to play the gamble with Mary. Ryota tells Mary that it is too soon without even explaining the rules to Yumeko. Mary looks annoyed to Ryota asking if he has a problem. Ryota backed down which Mary explains the rules to Yumeko. Yumeko became more interested in the game and tells Mary that she wants to tries it. Mary grabs Suzui's tag telling her to grab the chips, which he agrees to.
Once Ryota returns with the chips, Mary explains to Yumeko that per chip values 10,000 yen and to start, she prepared 120 chips. Ryota thoughts goes about the amount of money is being used in the gamble. Mary tells Yumeko that she lets Yumeko decide the amount of betting every set and asks her to use a set as a try. She then asks her how much she is going to try. Yumeko then starts with two chips. Ryota finds it so sad that Yumeko got involved in gambling this early, and explains that it is rare to be able to play all three hands and there's more irregular hands like "two rocks-one paper" as the player can only draw three cards out. As well if there are a lot of rocks in their hands, there's likely to be more rocks in the game. This means as their opponent is also likely to play the rock, then they'll play paper. The gamble is a game were the player read this kind of uncertainty out.
Mary akss Yumeko if she is ready to draw their cards simultaneously. Yumeko won the first round with rock while Mary showed scissors. Mary asks Yumeko if she gets the rules and continues. Yumeko then bets 50 chips which is she is betting 500,00 yen. Mary praises Yumeko for having a fighting spirit. The next round concludes to a draw and both has to play the next card. Yumeko shows scissors and Mary shows rock which Yumeko losses 50 chips to Mary. Yumeko reveals that she only had scissors in her hand and puts in 2 chips in the next turn. Yumeko won the turn and Mary asks how much she will bet for the next round. Yumeko then goes again with 50 chips. Mary agrees to it and it continue's to the new turn. It becomes a draw, which afterwards they have to show a new card were Yumeko loses round and 50 chips.
Mary noticed that there is something about Yumeko and tells her viciously if she got scared which is not surprising as Yumeko lost one million yen to Mary. Yumeko tells her joyfully that the real game starts now and repeats the same steps as before with two chips. Ryota is thinks that it is done, because there is too much differences in chips and that you cannot turn the table if they bet only small amounts because they got scared. Yumeko loses all her chips which worries Ryota about what will happen to her afterwards. Yumeko asks Mary if its alright to borrow additional chips.
Mary agrees to Yumeko's proposal which she shows secretly vicious thoughts thinking to her that she will walk in a bog from here on. Mary then asks Yumeko how much she wants to add. Yumeko tells Mary one thousand chips which is 10 million yen. This shocks the entire classroom which Yumeko tells Mary that she will bet it all on the next set.
Mary became shocked when she hears it and Yumeko is asking her if she has a problem with it as she never heard that there are any limits. Mary yells at Yumeko saying that there is a problem and wonders how they could bet on such a high amount. She then explains further that there's no way to bet that kind of sum without hard guarantee. Yumeko grabs her bag and goes inside it and shows 10 million yen in cash which shocks the entire classroom. Mary yells at Yumeko that she is insane betting 10 million yen in a single rock-paper-scissors game. Yumeko explains to Mary that it is the reason why it so good. She tells Mary that gambling is fun because it is insane and invites her to gamble like crazy.
Mary says that she cannot go along with this silliness and Yumeko in her turn tells her if she got scared. This angers Mary which she accepts Yumeko's challenge. Ryoto became shocked that Yumeko is gambling such a high amount of money. Mary then assures herself for the next win and tells Yumeko if she choose which card she would play. Yumeko tells Mary that she think she didn't notice, and tells the entire class if they are co-operating with Mary. Mary asks Yumeko on what she is basing it off, which Yumeko tells her that her method is clumsy and recalls her about the previous rounds and explains what she has discovered. She tells Mary that she won't be able to fool around anyone if she is not prepared to shed her own blood.
Mary tells Yumeko that she is disappointed with the false accusations Yumeko is telling her and asks her to stop with it and blaming her if she is trying to delay the gamble with such insinuations which Yumeko responds with "not at all". She tells Mary further that the real fun starts now. Shocked, Mary tells Yumeko to draw her card which Yumeko telling her viciously that she has cautioned her. Mary becomes annoyed and asks her if she is ready. Mary draws out paper and Yumeko draws out scissors which Yumeko wins the bet. Shocked, Yumeko tells Mary not to turn her eyes away from reality. Yumeko then looks forwards of her payment of 8.8 million yen from Mary which Mary asks Yumeko to give her a few days to give her the money.
Yumeko then leaves the classroom, with Ryota calling out for her. Ryota asks Yumeko how she knew Mary would draw our paper, which Yumeko explains everything to her. Afterwards, Ryota tells Yumeko that he will drop out of school, which Yumeko gives her the money to pay his debt as a thanks. From that, Ryota comes to an conclusion that Yumeko is a Compulsive Gambler.
- Ryota is the first character in the series that is shown to be used as a pet after losing from Mary Saotome.
- This chapter shows Yumeko's first day at Hyakkaou Private Academy.
- This is the first chapter that shows an original gambling game.
- This chapter explains the hierarchy of Hyakkaou Private Academy.
- Poker, it is seen early on the beginning of the chapter when Ryota was playing against Mary Saotome, and later on when Mary plays it with her fellow classmates.
- Vote Rock-Paper-Scissors, it is seen later in the chapter when Mary challenges Yumeko for a gamble. It is an original game made up by Mary.