Harumi Katayama is the younger sister of detective Yoshitaro Katayama and an aspiring actress who shares a home with her brother and their calico cat, Holmes. When a murder occurs at her acting troupe's after-show party, she becomes drawn into solving the case.

Sociable, energetic, and self-assertive, she stands in sharp contrast to her more timid brother. Harumi is free-spirited, wilful, and obstinate by nature, often taking initiative and displaying a keen interest in mysteries, especially those involving danger. She regularly pursues investigations independently, sometimes bringing Holmes along. Within their relationship, she offers loving pep talks to bolster her brother's confidence, reflecting her supportive yet strong-willed role in the family.