Yumeko Jabami is the main protagonist of Kakegurui. Yumeko is a transfer student at Hyakkaou Private Academy and a classmate of Ryota Suzui and Mary Saotome. Jabami's family is aligned with the family of Kirari Momobami and challenges her for Hyakkaou Private Academy.
Yumeko has pale skin, long black hair styled with a hime-cut and brown colored eyes. Her eyes are sometimes seen turning red in the anime and manga when she becomes excited. She wears the Hyakkaou Private Academy issued uniform; a red blazer with black trim lining the cuffs and collar, a white button up dress shirt, a dark pleated skirt, a tie and black/grey colored stockings. As well as the academy's issued footwear, brown colored loafers with black soles. She also sports red lipstick and has red painted fingernails (sometimes seen pink). She wears a ring around her left thumb.
It is hinted that she has low stamina as she was having difficulty in climbing five floors worth of staircases.
Jabami is shown to be a very upbeat and charismatic character, consistently attracting the affection or interest of those around her. Not only is she able to befriend those who previously disliked her, she is able to catch the attention of the entire student body. She does just that on her very first day of school and manages to continue doing so on nearly a daily basis. Every time she notices an oppenent cheating in games, her button gets switched to a bloodthirsty side of her, which is shown when her eyes turn red and frighten her opponents so much.
Though she is typically very friendly, she abstains from acting this way when she uses her perceptiveness to expose her classmates' cheating. She is capable of doing this even after only a couple of rounds of gambling, as she is easily able to decipher what strategies and tricks they are using to skew the game in their favor. While doing this, she may unabashedly condemn the other player and their ways, preferring honesty over politeness in such cases.
Despite the disadvantage she faces when others cheat, she does not concede. This is largely due to the fact that she does not seem to experience fear or anxiety from taking risks. In fact, it gives her a thrill. Because of this, she tends to take high risk bets in gambles, even if she has nothing to rely on but luck. Though this can sometimes end in her loss, she is emotionally unaffected by the results; she simply uses her loss or gain as a means to further indulge her gambling habit.
She seems to be fairly aware of her eccentricities and that her way of gambling doesn't follow normal conduct. As such she rejoices when people support her or encourage her, especially those close to her, greatly valuing their friendship, albeit in her own ways.
Kakegurui - Compulsive Gambler
Jabami transfers to Hyakkaou Private Academy and is shown around the school by her classmate Suzui. Suzui tells her about the school which she seems to be interested in. She became popular in her class were she is surrounded by her fellow male classmates. She is then challenged by Saotome to a gambling match which she accepts. She is shown to be losing after betting the match after betting 1 million yen during the first 2 rounds. Jabami then asks Saotome for another round to which Saotome agrees, when asked how much she's willing to bet, Jabami reaches into her school bag and produces stacks of yen that amount to 10 million, especially when Jabami puts 10 million yen on one round. Jabami then points out that Saotome rigged the game in her favor by having 21 out of 30 students place rigged cards that would tip the game into her favor. Jabami reveals that she noticed this patterns from the beginning and reveals Saotome's strategy to her. Jabami wins the gambling match leaving Saotome with a debt of 8.8 million yen which she tells Saotome that she her debt is already paid as their gambling match proved to be a fun ordeal. After that, Jabami leaves her classroom were Suzui calls out at her. He asks her how she knew about Saotome's rigging of the match to which she reveals to him how she deduced everything. She later gives him money to pay his debt that he has to Saotome as a thanks.
The next day it is seen that Jabami and Suzui are now friends. They enter their classroom to find Saotome's desk vandalised with the entire class smiling menacingly. Saotome blames it on Jabami because of her loss yesterday before lurking away to earn the money back. Suzui takes Jabami to the cafeteria where he explains the oppressive structure of the school, the student council and their gambling prowess, increasing her already growing love and excitement towards the school. Jabami is then invited to play Concentration against Itsuki Sumeragi. Jabami plays a close game but loses the 20 million yen given as a trial. Jabami demands a rematch and Sumeragi accepts under the condition that should Jabami lose, Sumeragi can remove her fingernails and toenails to add to her nail collection. Jabami realises her opponent was cheating and wins on the very first turn by remembering every card's patterns on the back. While Sumeragi is in shock, Jabami confronts her on the cheating and starts spewing out her excitement mixed with anger asking for another game where they both bet their nails. The confrontation caused Sumueragi to cry and decline Jabami's desire making the latter disappointed.
- Yumeko's surname: Jabami（蛇喰）can be translated to have any number of meanings; as the first kanji （蛇）can mean "snake/serpent" or "hard drinker" and the second kanji（喰）translates to "eat," "drink," or "to receive a blow." The likeliest translation of her surname is snake eater.
- Her first name: Yumeko（夢子）translates to "dream child."
- In the first volume, it was revealed that she lives alone and her parents have passed away. She also has an older sister that is currently being hospitalized.