Mari Watanabe is a prominent student at First High School, hailing from the Watanabe family within the Hundred Families lineage. Her family descends from General Watanabe Tsuna and occupies the hierarchy's lower tiers, affording them reduced political obligations and enemy scrutiny compared to higher-ranking families. Though not from the main branch, Mari stands out as her household's sole member possessing significant magical talent. She enrolled alongside Katsuto Juumonji and Mayumi Saegusa; their exceptional magical power and status as the school's strongest students earned the trio recognition as the "Triumvirate" or "Big Three." Physically, Mari is a young woman of average height (168 cm) with short black hair, brown-gold eyes, and handsome features. Her personality blends discipline, resilience, and outspokenness, coupled with a sporty, combat-oriented demeanor that secured her position as Public Morals Committee Chief. In this role, she maintained order and enforced rules, later delegating organizational and paperwork duties to Shiba Tatsuya after recruiting him. Subordinates occasionally used yakuza-tinged terms like "big sister" or "Boss" to address her, reflecting her leadership style, though she disliked such terms from peers. Mari shared a close friendship with Mayumi Saegusa, frequently collaborating on mischievous activities like teasing others. Her relationship with boyfriend Chiba Naotsugu reveals a contrasting softer side, where she displays more affectionate behavior. Magically, Mari specializes in versatile anti-personnel techniques rather than overwhelming power or speed. Her signature ability involves chaining multiple magics simultaneously, creating complex attacks difficult to counter. Key techniques include Dojigiri (Multiple Slash), a three-pronged sword attack utilizing thrown blades and gravity-accelerated pressure slashes developed through self-study of ancient texts with Naotsugu's assistance. She employs Pressure Slash, a weight-type magic extending blade reach via repulsion fields. Her Absorption-Type Magic combines chemical dust with oxygen conversion to induce hypoxia. She also manipulates airflow to deliver chemical effects like intoxication through scent and uses Commercial Aromatherapy for interrogation via scents that lower mental resistance or provoke emotional responses. Her combat skills include swordsmanship, though she ranks below specialists like Erika Chiba or Naotsugu. During the Nine Schools Competition, she competed in the Battle Board event but withdrew following an accident involving a Seventh High competitor, leading to Miyuki Shiba replacing her in Mirage Bat. After graduation, Mari entered the National Defense Academy as a temporary sergeant in the martial arts department, sharing an apartment near the campus with Naotsugu. She joined the "Sword Corps," a unit primarily composed of the National Defense Force First Division. She participated in operations like the search for Kudou Minoru after his transformation into a Paranormal Parasite, motivated by the corps' sympathies toward the deceased Kudou Retsu.