Emi Isuzu is a third-year student at Todo Academy and serves as the vice-president of the Executive Council, acting as the second-in-command to its formidable leader, Mitsuomi Takayanagi. Her family, the wealthy Isuzu clan, has a traditional role as the inner guard for the Takayanagi family, a duty she upholds as part of her background. Before her time on the council, she worked as a bodyguard for Mana Kuzunoha, often tasked with videotaping powerful and intriguing individuals associated with the Takayanagi family.

Emi is defined by her deep and unwavering respect for Mitsuomi, which borders on romantic love. This devotion makes her fiercely protective, especially given the heart condition that limits Mitsuomi's physical exertion. Her feelings for him also fuel a specific rivalry with Maya Natsume, for whom Mitsuomi still harbors lingering affections; Emi sees Maya as a rival she must surpass, though the manga later hints at a complex relationship between the two that possesses elements of mutual respect or strange friendship.

At first glance, Emi appears as a slender and exceptionally beautiful young woman, popular among the male student body. However, this appearance is a deliberate concealment of her most closely guarded secret. Her combat style revolves around her unique ability to contort and fold the mass of her naturally obese body, allowing her to compress it into a much slimmer form. Within the folds of this hidden mass, she can stash dozens of throwing knives and other small weapons, allowing her to seemingly pull blades from thin air during a fight. This secret technique has earned her the moniker The Black Blade. Exerting too much of her ki, a form of energy, forces her to revert to her true, larger form, revealing her secret to her opponents.

In her role on the Executive Council, Emi is a capable and strategic fighter. She personally confronts Maya Natsume in a restroom during a planned attack, managing to dislocate Maya's shoulder before being defeated. Her sense of duty is strong; she reprimands fellow council member Shinobu Kagurazaka for abandoning Mitsuomi and rushes to provide support when she can. The flashback arc reveals the origin of her deep loyalty to Mitsuomi. During that time, she was held hostage to provoke the deranged Shin Natsume. When Shin turned his aggression on her after defeating his captors, it was Mitsuomi who intervened. His attempt to save her resulted in the severe ki blast that caused his heart condition, an act for which she remains profoundly grateful and which cemented her devotion to him.

Emi is a descendant of a powerful lineage, originally from China, that produced generations of martial artists and bodyguards for the Takayanagi family. As the story progresses, she becomes a more supporting character, especially after Mitsuomi orders the council to cease direct attacks on the Juken club. Her strategic mind continues to show itself, particularly during the election tournament, where she employs devious tactics to try to eliminate the Juken club, motivated by her distrust of certain former allies like Bunshichi Tawara and her concern that the Executive Council cannot win otherwise. Her deep-seated fear is for Mitsuomi's life; when she learns of a plan that seems designed with his death in mind, she is overcome with grief, believing he is near his end. Despite being phased out of the central action later in the manga, she is identified as one of Mitsuomi's "true wings," a crucial part of his inner circle alongside fighters like Bob Makihara.