Masamunya Dokuganryū is a samurai cat distinguished by his white fur, black eyes, and a prominent eyepatch-like marking over one eye, visually inspired by the Japanese historical figure Date Masamune. He initially sets out to confront and defeat the vampire cat Nyanpire, perceiving him as a demonic threat. Their first encounter ends comically when Masamunya trips mid-charge, sustaining minor injuries. Nyanpire licks the blood from these wounds, unexpectedly sparking romantic feelings in Masamunya. This incident drives his primary motivation: an earnest but frequently clumsy pursuit of Nyanpire’s affection, including persistent marriage fantasies despite learning Nyanpire is male.

Masamunya displays a tsundere personality, alternating between bravado and vulnerability. He carries a sword but seldom wields it effectively, often tripping or stumbling during attempts to project strength. His self-proclaimed title "The One-Eyed Dragon" starkly contrasts with his combat ineptitude and daily clumsiness, reinforcing his comic relief role. Despite these shortcomings, he exhibits protectiveness, such as imposing a health regimen on Nyanpire during health scares and striving to please him during seasonal events like summer festivals. For Halloween, he cross-dresses, highlighting his willingness to embrace unconventional acts for Nyanpire’s approval.

His relationships extend beyond Nyanpire. He shows overt jealousy toward Nyatenshi, a fallen angel cat befriending Nyanpire, whom he views as a romantic rival. This rivalry sometimes escalates into non-serious conflicts. Masamunya also interacts with other characters like the bats Mori and Komori, though these dynamics remain less explored. A recurring theme involves his anxiety over mortality; in the series finale, he grapples with Nyanpire’s immortality limiting their time together, adding melancholy to his arc. His character development stays minimal, primarily anchoring the series through loyalty and comedic mishaps.