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 Suzui 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 in Hyakkaou Private Academy, Suzui shows her around the school. Later on, Jabami is being challenged by Saotome for a "Rock-Papers-Scissors" gamble were Jabami loses all her chips during the gamble, but the gamble continue's when Jabami 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 in his hands, he exposes them revealing them to be "full house". His opponent smiles viciously revealing a "royal flush" making the boy lose 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 attends 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 Jabami the school. Jabami is shown around the school and is impressed on how the school looks like. Then, Suzui asks her whether she knows anything about gambling. There, Suzui explains the situation of the school and asks her to be careful if she gets invited to participate. Jabami laughs sinisterly, saying that it is interesting.
Once the school day is over, Jabami is surrounded by her male classmates. This stirs the girl, who had beaten Suzui, who introduces herself to Jabami revealing her name to be Mary Saotome. She asks Jabami if she has the time to gamble with her. This peaked interests in her classmates.
Jabami asks her what game they are about to play. Saotome tells Jabami they are to play a game called "Vote Rock-Paper-Scissors" saying that it is an original game. This peaks interest in Jabami as she agrees to it to gamble with Saotome. Suzui tells Saotome that it is too soon without even explaining the rules to Jabami. Saotome looks annoyed asking Suzui if he has a problem. Suzui backs down which Saotome explains the rules to Jabami. Jabami becomes more interested in the game and tells Saotome that she wants to tries it. Saotome grabs Suzui's tag telling him to grab the chips, which he agrees to.
Once Suzui returns with the chips, Saotome explains to Jabami that per chip values 10,000 yen and to start, she prepared 120 chips. Saotome tells Jabami that she will let her to decide the amount of betting every set and asks her to use a set as a try. Jabami then starts with two chips. Suzui finds it sad that Jabami 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.
Saotome asks Jabami if she is ready to draw their cards simultaneously. Jabami won the first round with rock while Saotome showed scissors. Jabami then bets 50 chips, 50,000 yen, in the next round. Saotome praises Jabami for having fighting spirit. The next round concludes to a draw and both has to play the next card. Jabami shows scissors and Saotome shows rock resulting in Jabami losing 50 chips. Jabami reveals that she only had scissors in her hand and puts in 2 chips in the next turn. Jabami won the turn and Saotome asks how much she will bet for the next round. Jabami then goes again with 50 chips. Saotome agrees to it and it continues to the new turn. It becomes a draw, which afterwards they have to show a new card were Jabami loses round and 50 chips.
Saotome noticed that there is something about Jabami and berates her viciously if she is scared which is not surprising as Yumeko lost one million yen to Mary. Jabami tells her joyfully that the real game starts now and repeats the same steps as before with two chips. Suzui 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. Jabami loses all her chips which further worries Suzui about what will happen to her afterwards. Jabami asks Saotome if its alright to borrow additional chips.
Saotome agrees to Jabami's proposal which she shows secretly vicious thoughts thinking to her that she will walk in a bog from here on. Saotome then asks Jabami how much she wants to add. Jabami tells Saotome one thousand chips which is 10 million yen. This shocks the entire classroom with Jabami telling Saotome that she will go all in on the next set.
Saotome became shocked when she hears it with Jabami asking her if she has a problem with it as she never heard that there are any limits. Saotome yells at Jabami 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. Jabami grabs her bag and goes inside it and shows 10 million yen in cash which shocks the entire classroom. Saotome yells at Jabami that she is insane betting 10 million yen in a single rock-paper-scissors game. Jabami explains to Saotome that it is the reason why it so good. She tells Saotome that gambling is fun because it is insane and invites her to gamble like crazy.
Saotome says that she cannot go along with this silliness and Jabami in her turn tells her if she got scared. This angers Saotome making her accept Jabami's challenge. Suzui became shocked that Jabami is gambling such a large amount of money. Saotome then assures herself for the next win and tells Jabami if she choose which card she would play. Jabami tells Saotome that she think she didn't notice, and tells the entire class if they are co-operating with Saotome. Saotome asks her what she is basing it off, which Jabami tells her that her method is clumsy and recalls her about the previous rounds and explains what she has discovered. She tells Saotome that she won't be able to fool around anyone if she is not prepared to shed her own blood.
Saotome tells Jabami that she is disappointed with the false accusations Jabami 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 Jabami responds with "not at all". She tells Saotome further that the real fun starts now. Shocked, Saotome tells Jabami to draw her card which Jabami telling her viciously that she has cautioned her. Saotome becomes annoyed and asks her if she is ready. Saotome draws out paper and Jabami draws out scissors which meant Jabami wins the bet. Jabami tells the shocked Saotome not to turn her eyes away from reality. Jabami then looks forwards of her payment of 8.8 million yen from Saotome which she asks Jabami to give her a few days to pay her the money.
Jabami then leaves the classroom, with Suzui calling out for her. Suzui asks Jabami how she knew Saotome would draw our paper, which Jabami explains everything to him including his role in deciding which card for the others to put in. Afterwards, Suzui tells Jabami that he will drop out of school, which Jabami gives him the money to pay his debt as a thanks for making the game fun. From that, Suzui comes to an conclusion that Jabami 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.