Mio Hoshiyama is a friend of protagonist Musashi Kazama, completing a close-knit trio alongside Suguru Ohmori. They meet after Musashi relocates from Tokyo to a rural setting. Mio actively participates in fishing adventures and confronts antagonists employing unethical methods.

In the sequel series *Grander Musashi RV*, set after a time skip, Mio acquires an upgraded fishing rod comparable to Musashi's original specialized equipment. This occurs concurrently with Suguru receiving similar gear and Musashi's own rod being enhanced.

The sequel introduces numerous rivals pursuing legendary lures called Legenders. Amid this expanded narrative scope, Mio and Suguru experience reduced prominence due to the increased focus on these new competitors and their distinctive lures.

Interactions between Mio and Musashi consistently feature elements of romantic tension throughout both the original series and *RV*. The manga and initial anime adaptation present this dynamic with greater emphasis, though no definitive romantic relationship develops within the official continuity.