Mitsuyoshi Yagyu is a 15-year-old third-year student at Daiwa Institute, born March 16. She stands 156 cm tall with brown hair, brown eyes, blood type O, and measurements of B75-W53-H81 cm. Designated an A-rank fighter, she is the successor to the historical swordsman Yagyū Jūbei Mitsuyoshi.

Her combat style centers on swordplay. Confident in her abilities, she deliberately wears an eyepatch during fights as a self-imposed handicap, not due to any eye injury. She first appears challenging Shiryuu Chou-un to a duel, seeking Chou-un's Magatama. During their clash, Mitsuyoshi's eyepatch is severed, revealing Chou-un wielded an inferior sword. Acknowledging Chou-un would have won with her proper weapon, Mitsuyoshi postpones the duel out of newfound respect.

Following the fight, Mitsuyoshi forgets her wallet and borrows money from Chou-un. This leads her to join Chou-un at Seito Academy. Mitsuyoshi reveals she, along with other western warriors, was ordered by Himiko to collect Magatamas, though Mitsuyoshi remains unaware of Himiko's ultimate goal.

Her background includes a prior acquaintance with Ekitoku from middle school. To repay an old favor to him, she allies with him against Inshun Hozoin. During this battle, Inshun poisons her.