The following contains spoilers for Kakegurui Twin Episodes 1-6, now streaming on Netflix.
Mary Saotome isn't exactly known to have high moral standards when it comes to gambling. As an overconfident player who always plays to win at all costs, she employs more cheating methods than she plays fair. She is not even beyond throwing other students under the bus once they no longer prove advantageous to her and is pretty cruel to her own "house pet," Ryota Suzui. Despite possessing these negative traits, however, Mary is still known to have a moral compass, which Kakegurui Twin showcases.
In the main Kakegurui: Compulsive Gambler series, Mary interacts with various students, including her one-time rival Yumeko Jabami. In Kakegurui Twin, however, she is given her own supporting cast in the form of Tsuzura Hanatemari -- a classmate she's known since primary school -- and Yukimi Togakushi, another Hyakkaou Private Academy student who loses her gambler den to Mary. Both of these characters help explore Mary's softer side, such as her need for companionship as well as her compassion for others; two aspects of her character that don't get much focus in the main series.
One surprising development for Mary in Kakegurui Twin is her hatred of the Hyakkaou Private Academy caste system. More specifically, she hates the dehumanizing "house pet" system the Student Council president, Sachiko Juraku, established, which is ironic considering she had a "house pet" of her own in the main series. A year prior to meeting Ryota, however, Kakegurui Twin establishes that Mary was herself fearful of this system, and especially of the Student Council. Interestingly, the person who makes her hate the system is not Sachiko herself but her friend Tsuzura.
Tsuzura is the first character introduced Kakegurui Twin who is alluded to but absent in the main series. When Tsuzura introduces herself, she establishes right away that she is a poor gambler and that her losses led to her becoming a "house pet" -- the lowest status a student can occupy in the school's caste system. The "house pets" are identified with name tags that are either labeled "Fido" for men or "Mittens" for girls. Tsuzura is seen wearing the "Mittens" tag and is subjected to bullying by her classmates. When Mary transfers into her school, Tsuzura sees that she is struggling to make friends and asks Mary to befriend her. This comes off as strange to Mary, as she is not yet used to the school's caste system.
After Tsuzura helps Mary win a gambling match she initially lost in the first round by sporting her some money, Mary starts to pity her former classmate and befriends her. After winning some money from the second round, Mary along with Tsuzura decides to target the least popular gambling den to take over and start earning more money. In this case, they target the library currently maintained by Yukimi, who also happens to be greatly indebted to Sachiko. After learning this detail, Mary uses this opportunity to win Yukimi's gambling den through a simple probability game that she wins through cheating. While this forces Yukimi to clear out the library, she still asks Mary to let her continue using it under her supervision, which Mary allows, facilitating a friendship between them.
With Yukimi and Tsuzura now helping Mary run her gambling den, Mary decides to shift her focus toward ending the school's dehumanizing caste system. To this end, she prioritizes ending Tsuzura's status as a "house pet" by earning a set amount of money that will both pay off her debt and also earn them profit. However, Mary isn't the only one looking to end the school's cast system. One of Sachiko's Student Council executives, Aoi Mibuomi, also wants to end it on his own terms and tries to recruit Mary to his cause. To ensure that she will at least consider it, he summons her along with her friends to his office, and all three girls are talked into participating in a couples' dating game.
During the couples' dating game, Mary is profoundly distrustful of everyone on her team (including Yukimi) except for Tsuzura, with whom she shares her cheating strategy to secure a win. To Mary's frustration, however, one of her teammates also cheated and secured a loss for the entire group. Sachiko uses this opportunity to gaslight Mary into doubting Tsuzura's loyalty, which works very briefly. Once Mary works out who the real traitor is and how the game is rigged, she secures a win for her group. This win helps to restore Mary's trust in Tsuzura and learns to trust Yukimi as well.
When Mary is formally invited by Aoi to join his Full-Bloom Club, she declines membership with the excuse that she needs to give it some thought. Mary instead chooses to trust her two friends and encourages them to side with neither Aoi nor Sachiko, but to use them whenever it becomes advantageous for them to do so. This strategy proves useful to all three girls when Aoi's fiancée, Sakura Miharutaki, challenges them to a scavenger hunt. They are only able to win against her by trusting each other and strategizing together.