Yoshino Naganohara is Mio Naganohara's older sister, attending college. She frequently dons costumes to play unsettling pranks on Mio, chasing her and creating tension between them. Despite her academic standing, Yoshino exhibits immaturity through these persistent antics, which Mio finds excessive and irritating.

Yoshino possesses exceptional, natural talent in kendo, recognized as a genius. She effortlessly won a national tournament, claiming practice diminishes her strength. This attitude frustrates Mihoshi Tachibana, a dedicated practitioner who trains rigorously yet cannot match Yoshino's innate skill. Their interactions underscore Yoshino's casual dominance, as seen when Mihoshi unsuccessfully attempts to strike her during a walk, accidentally hitting their assistant instructor instead.

A flashback reveals Yoshino and Mio were once close, bonding over shared kendo practice at the dojo where Mio first met Koujirou Sasahara. Their dynamic shifted over time as Yoshino's pranks became a source of conflict. Yoshino's actions stem from playful mischief, lacking clear further motivation. Her role remains consistent as a supporting figure defined by her kendo prowess and antagonistic sibling behavior, with no significant character development or altered relationships depicted across official media.