Irina Pouffanovitch is a professional assassin introduced as a new English teacher assigned to Class 3-E. She is a young woman of Russian origin, fitting a certain stereotype with her tall, slender figure, pale skin, long curly blonde hair, and blue eyes. Her primary mission, for which she is hired by the authorities, is to eliminate the target using her position as a language teacher as a cover.

As an assassin specializing in infiltration, Pouffanovitch possesses excellent acting skills and is characterized by a ruthless determination to achieve her objectives, viewing her targets with an unyielding focus. She often relies on her appearance and the art of seduction as tools to manipulate and get close to her victims. Despite her dangerous profession, outside of work she reveals a surprisingly normal and clumsy side, having lost touch with everyday life due to her high-end hitwoman lifestyle. This can sometimes make her attitude seem childish or unprofessional, much to the dismay of her colleague and handler, Karasuma.

In the story, her initial role is as an antagonist of sorts, attempting to prove her skills by killing the homeroom teacher while clashing with the students who are also trying to accomplish the same goal. However, she eventually settles into the role of a faculty member, teaching the students of Class 3-E practical skills in conversation, deception, and mental resilience. A key relationship is formed with Karasuma, her handler and fellow teacher; their close professional partnership eventually deepens, leading to marriage and the birth of a daughter. Over the course of the series, her character develops from a seemingly frivolous and self-serving femme fatale to a more dedicated, if still unorthodox, instructor who genuinely contributes to the students' unconventional education. Her notable abilities include her prowess as a professional killer, expertise in foreign languages, and a high degree of proficiency in acting and seduction.