Kaze Hikaru Vol 1

Into this fierce milieu steps Kamiya Seizaburo, a young would-be warrior who, though lacking in combat experience, possesses a fiery enthusiasm to both aid the Mibu-Roshi in their mission and to avenge his wrongfully murdered family. One of the Mibu-Roshi's most gifted (and immature) swordsmen, the legendary Okita Soji, agrees to take Seizaburo under his wing. What no one suspects, least of all Soji, is that Seizaburo is actually a girl named Tominaga Sei in disguise! -- VIZ Media