Kakegurui Episode 12 can be described with one single word: Filler.
This episode absolutely does not contribute to the story at all, with Episode 11 being fine as the final episode.
The production team must have been like: how do we add another episode to the anime to make Yumeko battle it out with Kirari? Oh, of course. Let`s add a gambling match for this pair of rivals with a crappy setting. Oh, and let`s have Suzui be the MVP of this gambling match. The match itself was useless.
Short Summary of the episode: face-off between Student Council President Momobami Kirari and Jabami Yumeko, with Suzui in the game because… why not? Gamble ended in a draw thanks to Suzui power of believing, like how Yugi believes in the heart of the cards.
Good things we can extract from this episode is that the bet Kirari and Yumeko made were reasonable and could potentially be used in the manga. Also, it was an episode to spark doujinshi material between Suzui and Yumeko.
Thanks for reading!