Naomi Tanizaki is the adopted younger sister of Junichiro Tanizaki and an office clerk at the Armed Detective Agency. She is a young girl who always wears a sailor school uniform and is known for her bold, strong-willed personality. Unlike her brother and many of her colleagues, Naomi possesses no supernatural abilities, but she never hesitates to involve herself in cases, particularly those that concern her brother's well-being. Her caring and protective nature toward Junichiro is a defining trait, and she is often described as quick‑thinking and resourceful despite lacking powers.

Her motivations are closely tied to her loyalty to the agency and especially to her brother. She works as a secretary part‑time, supporting the front‑line members from the office. In the story of Dead Apple, Naomi appears as a supporting character, present among the Armed Detective Agency members during the crisis caused by the mysterious fog. Her role is not central to the plot, but she contributes to the agency's efforts in her capacity as a clerk and occasional field supporter.

Naomi's key relationships include her deep bond with Junichiro, whom she adores and fiercely protects, as well as her professional interactions with fellow agency members such as Atsushi Nakajima and Doppo Kunikida. Her development across the series is subtle; she remains a dependable and cheerful presence, showing her growth through her consistent willingness to stand by her brother and the agency even in dangerous situations. Her most notable trait is her lack of an ability, which makes her reliance on courage and determination stand out in a world of gifted individuals.