Tetsuo Takahashi teaches biology at Shibasaki High School. He possesses a tall, muscular frame, spiky black hair, permanent stubble, and notably small "rice-shaped" eyes noted as a family trait. He typically wears a white lab coat over casual attire like red t-shirts and black slacks. His imposing physique contrasts with his gentle nature, earning him student and colleague nicknames such as "Iron Man" (a pun on his first name meaning "iron") and "Gorilla Man."

He graduated from Musashino University of Science, majoring in biology. His thesis proposal focused on demi-human research, but it was rejected due to societal sensitivities and insufficient government support. Reluctant to enter corporate work, he obtained a teaching certification and joined Shibasaki High. Initial ambivalence toward teaching shifted when he encountered demi-human students: vampire Hikari Takanashi, snow woman Yuki Kusakabe, and dullahan Kyōko Machi, alongside succubus math teacher Sakie Satō. This fulfilled his long-standing interest in observing demi-human physiology and social integration directly.

Takahashi blends scientific curiosity with deep empathy. He interviews the demi-humans to document their experiences but consistently prioritizes their comfort and well-being over research goals. He maintains professional boundaries despite the girls' affections—Machi’s schoolgirl infatuation and Satō’s romantic interest. He addresses sensitive topics like vampire blood urges or succubus seduction effects respectfully. He suppresses personal reactions to Satō’s aphrodisiac aura to prevent distressing her, though physical contact with her causes him intense internal strain. His reliability makes him a confidant for demi-students facing bullying or isolation, though he later questions if his help might impede their self-sufficiency.

Key relationships mark his development:
- With Hikari, he allows the informal "Tetsu-sensei" and discusses vampire biology openly, building trust despite her mischief.
- He supports Machi’s dullahan needs, such as securing permission for backpack use, while kindly deflecting her advances.
- For Yuki, he scientifically deduces her snow-woman limitations to ease her fear of harming others, aiding her socialization.
- His bond with Sakie evolves from colleagues to mutual romantic interest. Initially insecure about reciprocating due to her powers, he eventually confesses and begins dating her, lessening his earlier emotional suppression.

Throughout the series, Takahashi shifts from a researcher driven by scientific curiosity to an advocate promoting demi-human acceptance. He reconciles his role by encouraging independence while remaining a steadfast mentor. His personal life progresses through his relationship with Sakie, reflecting a gradual embrace of emotional vulnerability alongside his inherent kindness and integrity.