Isuzu Tsumuhana lives in the same Jouga mountain town apartment complex as Hiroshi Kuzumi. Her family has deep roots in Jouga, having previously lived in the old town sector. She possesses a tomboyish, energetic, and openly affectionate nature, instantly forming a strong attachment to Hiroshi when he arrives. This attachment translates into constant physical closeness, like clinging to his arm, and frequent enthusiastic interactions that often leave him unsettled. Her cheerful demeanor spreads good humor, making her popular and easy to befriend among classmates.

She holds her family in high regard, especially her older brother, the university student Issei Tsumuhana, expressing sadness at the thought of him moving out due to his popularity. Tragically, Issei later succumbs to his predatory "Godman" instincts and attacks Hiroshi, leading to his execution by Nemuru Kushinada. Following her brother's death, Isuzu herself begins to transform and loses control, attacking Hiroshi. Unlike Issei, she manages to halt her attack and regain her senses, thereby avoiding execution.

Physically, Isuzu has blonde (ochre) hair and brown eyes. She wears her school uniform and a distinctive bracelet with a small bell on her left wrist. She harbors a notable fondness for sweet foods, particularly cake and ice cream, which shapes how she prefers to explore Jouga. Her close relationship with best friend Kaname Asagiri endures, though Kaname finds Isuzu's sudden infatuation with Hiroshi puzzling. Her development centers on grappling with her dangerous heritage, culminating in her struggle to resist her predatory nature during the attack on Hiroshi.