Tamotsu Yanagisawa, an investigator with the Tokyo police, maintains a long-standing professional connection to Rin Asougi. He serves as one of her key confidential sources, repeatedly sharing sensitive police records despite the substantial risk to his career. Yanagisawa provides this assistance without expectation of favors or compensation.

He frequently attempts to arrange discreet meetings to minimize exposure, though these efforts often fail. His inconsistent methods or Rin's tendency to loudly use his nickname "Tamo-chan" in public spaces like restaurants inadvertently compromise his secrecy. Aware of the dangers he faces, Rin avoids intentionally endangering him and only seeks his help when other information avenues are exhausted.

A flashback prior to his death reveals an intimate moment shared with Rin, confirming a past romantic relationship between them. This history partially explains his continued willingness to assist her despite the personal and professional hazards. Yanagisawa's involvement includes facilitating Teruki Maeno's introduction to Rin during the Ruon case investigation. His story concludes with his death, during which he recalls this past intimacy with Rin.