Matsumoto is an elderly turtle who originated as a stone statue within an isolated island's forest. He awoke when the seal on the malevolent entity Sedna shattered, coinciding with the arrival of sea dwellers Marin and Urin. They were searching for a discarded ring belonging to island resident Kanon. This unsealing granted Matsumoto the power of human speech and propelled him into an active role guiding subsequent events.

Following his awakening, Matsumoto took residence in Kanon's home and became a mentor to both Marin and Kanon. He revealed Marin's identity as the Priestess of the Sea and Kanon's as the Priestess of the Sky, guiding their duties against Sedna and the corrupted sea creatures. He also possessed the practical ability to generate fireballs required for one of Kanon's spells, directly supporting their magical combat capabilities.

Matsumoto advocated traditional methods inherited from past generations of priestesses to confront Sedna. This approach emphasized forceful containment, even at the potential cost of the host's life. Consequently, when Urin succumbed to jealousy and darkness, becoming Sedna's vessel, Matsumoto declared her beyond saving. He insisted Marin and Kanon focus solely on defeating Sedna through force, dismissing their desire to rescue Urin as a perilous distraction.

His perspective and insistence on this traditional path created fundamental conflict with Marin and Kanon, who remained determined to save Urin. Their resolve was tested when Sedna absorbed their magic and destroyed their transformation stones. Ultimately, Marin and Kanon discovered Sedna's true nature as the embodiment of humanity's unacknowledged collective sorrow. By helping the islanders accept this sorrow, they achieved peace without destroying the host, resolving the conflict contrary to Matsumoto's initial guidance.

Visually, Matsumoto possesses a traditional turtle appearance featuring a prominent mustache and a hat.