Sunao Sumeragi, sole heir to the Sumeragi Dojo and its undefeatable sword style, possesses striking red hair and eyes. She favors simple attire like sleeveless shirts with jeans or shorts, alongside school uniforms. Her personality is intensely straightforward, hot-blooded, and impulsive, often acting rashly without forethought and displaying a short temper that leads to regrettable actions. She is bluntly honest, expressing her thoughts openly even when perceived as rude, and holds deep respect for authority figures like gods of the land. A profound fear of loss, stemming from her brother Suou's traumatic death, fuels her drive to become stronger primarily to protect those she cares about. Trained in swordplay from childhood alongside her idolized older brother Suou, Sunao blamed his tsugumomo partner, the katana Kotetsu, for his weakness after he died fighting an irregularly powerful amasogi. Seeking revenge, she demanded the god of the land Honoka grant her a powerful tsugumomo, but was refused due to inadequate skills. Ignoring this, she took Kotetsu and pursued her brother's killer, only to find it slain by Kanaka Kagami. Unsatisfied, Sunao attacked Kanaka but was swiftly defeated. This sparked a years-long rivalry where Sunao repeatedly challenged Kanaka to duels but never landed a single blow. She later enrolled at Tsuzura Temple for exorcist training and eventually passed Honoka's test to become Komiya's official exorcist, demonstrating her ability to block Honoka's "Flame of the Heavens" with her ultimate technique, Hibakiri. Her abilities encompass expert swordsmanship and refined spirit power control, enabling techniques like Ground Slash (Chibashiritsu), Air Slash (Karadachi), and Double Claw (Nisō). As Kotetsu's wielder, she leverages his energy-storage capacity to enhance her combat speed and power. Hibakiri exploits Kotetsu's energy buildup for rapid consecutive strikes. Initially viewing tsugumomo as mere tools and treating Kotetsu with disdain, Sunao's growth was limited. After losing a duel to Kazuya Kagami—where her overconfidence caused Kotetsu's hilt to break—she underwent significant development. She apologized for her behavior, began treating Kotetsu as a partner, and developed the Shadow Sword (Kagedachi) technique, using spirit power to create hidden attacks. Sunao's relationship with Kazuya evolved from rivalry to romantic interest. Her mother enforced an arranged marriage after her duel loss, citing honor restoration. To deceive her mother, Sunao staged a consummation with Kazuya using her own blood, but the ruse failed, resulting in punishment. Her feelings for Kazuya grew, often manifesting as embarrassment or tsundere-like behavior. She also became more open to spiritual intercourse, initially viewing it as lewd but later participating with Kazuya for power manipulation training, even displaying dominance during sessions. Her complex relationship with her mother involves deep-seated fear and obedience, as she is largely incapable of defying maternal orders.