Tomizoro "Tommy" Tengai serves as Shinichi Kuruma's loyal companion throughout their journey. He is the brother of Maria Tengai, whose "blood of Maria" holds significance for divine resurrection within the series. Tommy possesses extrasensory perception abilities, though dimmer than his late sister's, and his human limitations contrast with enhanced characters, making him a steadfast ally.

His past is marked by personal tragedy stemming from the "Great Destruction," catastrophic earthquakes that devastated Shinjuku, arising from earlier conflicts involving Shinichi and Maria. Tommy's premonitions warn of dangers, such as Shinichi's association with Ayaka Sanders, fearing her presence might trigger tragedy akin to Maria's demise. Despite these warnings, he remains committed to aiding Shinichi and protects Ayaka despite his misgivings.

Bound by loyalty and shared loss with Shinichi, Tommy actively confronts genetically altered adversaries and the threat posed by the "Doctors." He offers strategic assistance in battles, using his limited psychic insights to guide the group. His protective instincts extend to Ayaka, even as he questions her link to Shinichi and the cycle of violence.

Tommy acts as Shinichi's moral anchor and pragmatic counterpart. He navigates conflicts relying solely on human resilience and deductive skills, without physical transformations. His presence embodies themes of family bonds and the consequences of scientific experimentation, highlighted through his connection to Maria and the legacy of her sacrifice.