Mina Tatebayashi is a five-year-old half-mermaid, half-vampire youkai residing in Sakurashinmachi and the twin sister of Kana Tatebayashi. Following their parents' deaths—a vampire police officer and a mermaid—shortly after birth, Mina and Kana came under the guardianship of Shidou Mizuki, who harbors guilt over failing to protect them.

Mina possesses silver or light grey hair and blue or petrol-colored eyes, often sporting a white jacket or hoodie. Her perpetually raised eyebrows create a consistently angry expression. Exposure to water triggers temporary physical maturation due to her dominant mermaid heritage, transforming her appearance into a teenager or young adult, including increased bust size in some depictions.

Her personality is strong-willed, extroverted, and energetic. She speaks in a Kansai dialect, behaving in a brash, outspoken, demanding, cheeky, or bossy manner. Mina enjoys provoking others and expresses affection by jumping onto trusted adults' shoulders. She exhibits "old man-like" traits, favoring Japanese sake as her favorite food. Mina is straightforward, impatient, and short-tempered, resorting to rough language and actions when angered, and is less adept at intellectual tasks than her calmer twin.

Within the Tsuki ni Naku storyline, Mina becomes a central target for the werewolf Rokkaku, who seeks to sacrifice her for immortality. With the Hiizumi Life Counseling Office members absent, Shidou Mizuki intervenes to protect her. This confrontation highlights Mina's vulnerability and significance within her adoptive family, relying on her survival and recovery abilities.

Her hybrid heritage grants distinct traits: water exposure induces rapid physical maturation, while she possesses enhanced endurance and accelerated healing, recovering from injuries fatal to humans—abilities stemming from her vampiric and mermaid lineage.

Her name reflects this heritage: "Mina" combines "water" (水) and "apple tree" (奈), while "Tatebayashi" means "mansion hall" (館) and "forest" (林).