Jun'ichirō Tanizaki is an 18-year-old Armed Detective Agency member possessing a slim build, pale skin, hazel eyes, and orange hair parted down the middle, accented by a distinctive light grey bobby pin on the left side and matching earrings. His typical outfit includes an oversized off-white V-necked sweater featuring a black collar stripe, layered over a black tank top, light blue pants, brown shoes, and a red sweater tied around his waist. During the agency's framing as terrorists, he adopted a white hood suit and later wore military police riot gear with black boots and gloves.

Personality-wise, Tanizaki presents as one of the agency's most calm and gentle members, frequently polite and quick to apologize when perceiving fault. He describes himself as mediocre in combat and prone to nervousness, earning the nickname "wimpiest in the east" from colleagues. This self-assessment contrasts with his demonstrated aptitude for reconnaissance and adaptability in undercover operations. His moral compass becomes unstable when his younger sister Naomi is threatened, leading him to prioritize her safety above societal ethics, agency reputation, or personal pride; he explicitly states willingness to "burn the world" for her and displays uncharacteristic violence towards those endangering her.

Previously living as a student, Tanizaki now resides with Naomi, though their parents' whereabouts or past remain undisclosed. His entrance exam into the agency was notably traumatic and severe, resulting in repressed memories he avoids revisiting. Outside detective work, he demonstrates cooking skills, preparing dishes like Hayashi rice.

His ability, Light Snow, generates localized illusions by manipulating light refraction within a snowy visual effect. This non-combative power excels in stealth operations, surveillance, and misdirection by overwriting backgrounds or creating false imagery. The ability's name references Tanizaki Jun'ichirō's novel "Sasameyuki" (translated as "The Makioka Sisters").

Tanizaki's development unfolds across key arcs:
- **Port Mafia Arc**: He participates in Atsushi Nakajima's entrance exam by posing as a bomber holding Naomi hostage. When Naomi shields him from gunfire, he violently retaliates against Ichiyō Higuchi until Ryūnosuke Akutagawa intervenes, requiring recovery via Akiko Yosano's ability.
- **Guild Arc**: After Lucy Maud Montgomery traps Naomi in a dimensional space, Tanizaki panics and recklessly pursues her. He collaborates with Atsushi to free captives, using Light Snow to deceive Lucy. Later, when Guild members threaten Naomi during a mission, he intentionally causes a truck collision to protect her, declaring morality irrelevant to her safety.
- **Cannibalism Arc**: Following an attack on agency president Yukichi Fukuzawa, Tanizaki volunteers to surrender to the Port Mafia as part of Ranpo Edogawa's strategy. Captured by the Black Lizard, he feigns cooperation but turns hostile when they mention Naomi. Using a switchblade materialized by Doppo Kunikida's ability, he escapes restraints, assaults his captors with offensive illusions, and attempts to assassinate Ōgai Mori before Kōyō Ozaki critically injures him until Kyōka Izumi intervenes.
- **Hunting Dogs Arc**: After the agency is framed for terrorism, Tanizaki flees with Kunikida. When Kunikida sacrifices himself to expose the truth, Tanizaki hides in a deserted village arranged by the mafia. He later reunites with agency members via Edgar Allan Poe's ability and is captured by Ōchi Fukuchi during an airport mission.

In the comedic spin-off "Bungo Stray Dogs Wan!", Tanizaki appears without significant backstory expansion, retaining his established gentle demeanor, close bond with Naomi, and occasional embarrassment at her antics.