Yumeko is a mysterious transfer student to Hyakkaou Private Academy who initially appears modest, but shows a maniacal passion for high stakes gambling, as shown when her eyes glow red, which they often do. In spite of her dislike for games where the outcome is predetermined, she can quickly deduce how such games are being manipulated in her opponents' favor and, by doing so, she devises strategies that more often than not turn the tables on them.