Kensuke Matsui serves as a sword instructor and a samurai in the service of a local lord during the early Edo period. He is a man of considerable standing within his domain, respected quite a lot by his students who look up to him as a skilled and experienced master of the blade. His lord assigns him a critical and sensitive mission: to assist the legendary swordsman Musashi Miyamoto and to discreetly deal with Iemon, a former retainer who has become a traitor while investigating strange deaths in a village.

Initially, Matsui presents himself as a calm, capable, and reliable leader for the group of samurai sent on this mission. He speaks thoughtfully about his comrades, acknowledging that while some like Gensai may not be the most outstanding fighters, each possesses their own unique strengths. He also provides insight into Iemon, describing him as an exceptionally intelligent man who, had he been born in the chaotic Sengoku period, would likely have achieved great things. This analysis shows that Matsui is a perceptive observer of character and understands the gravity of the situation they are walking into.

However, this composed exterior hides a treacherous core. Matsui is secretly a traitor who has aligned himself with Iemon's cause. When Musashi sees a mark on Matsui's arm, a symbol identical to one found on a spy sent back by Iemon, Matsui drops all pretense. He confesses his betrayal openly, revealing that his true motivation is not loyalty to the lord but a personal belief in the world Iemon is trying to create, which he finds far more interesting than the current one. This moment uncovers a deep-seated ambition and a desire for a different, more exciting era of chaos and opportunity.

Upon revealing his treachery, Matsui undergoes a startling transformation. The reserved instructor vanishes, replaced by a man who is viscerally excited. His expression becomes one his students have never seen before, full of a fanatical thrill. He expresses exhilaration at the prospect of a country samurai like himself facing the legendary Musashi in a duel, treating the life-or-death struggle as a long-awaited and fascinating opportunity. This eagerness underscores a hidden arrogance and a desire to test himself against the very best, seeing the fight not as a mission but as a personal challenge.

In the narrative, Matsui serves as an early antagonist whose role is to establish the pervasiveness of Iemon's betrayal and the dangerous allure of his vision. His confrontation with Musashi is swift and brutal, demonstrating the vast gulf in skill between a respected dojo instructor and a true master swordsman. Musashi ends the duel in a single, devastating motion, cutting both of Matsui's hands before delivering a fatal slash. In his dying moments, Matsui's final words are not of regret or anger but of detached curiosity, commenting that Musashi's fighting style is quite interesting, a last glimpse of the analytical and fascinated mind that led him to betray his lord.

Regarding his fighting abilities, Matsui is a proficient swordsman and an experienced warrior, having participated in the Battle of Sekigahara, which grants him a veteran's understanding of combat. He is confident enough to challenge Musashi Miyamoto, a man whose legendary status he acknowledges. However, despite his confidence and skill, he is utterly outmatched. The actual fight lasts only seconds, with Musashi disarming and dispatching him with contemptuous ease, showing that while Matsui is competent, he is not in the same league as the greatest warriors of his age.