Ranmaru Yatsu operates as a police officer in the Shinjuku Police Department's Third Materials Compiling Group (Comps-3), directly reporting to Tsubaki Iwanome. He primarily conducts investigations partnered with Danjou Tozawa. His physical appearance features a slim build, short light hair, and habitual indoor use of sunglasses. Standard attire includes light-colored trousers, a double-breasted blazer layered over a darker shirt, and a signature necktie patterned with vertical columns of black stars—a motif echoed on accessories like his phone case. Personality displays surface-level confidence and assertiveness during confrontations, though he voices loud complaints when wronged and reacts visibly to taunts. Underlying this, he exhibits unwavering loyalty to Iwanome while retaining the willingness to challenge his superior with uncomfortable truths. In combat, he specializes in cane techniques, holding the top rank in individual combat skills within the Shinjuku Police Department. This is substantiated by his first-place finish in the Metropolitan Police Department’s arrest technique competition. His background includes a familial tie to Kochou Eightport, identified as his younger sister. The name "Ranmaru" (蘭丸) derives from "orchid" (蘭) and "circle/purity" (丸), while "Yatsu" (八津) combines "eight" (八) and "harbor" (津). Within the narrative, he formally debuts during Comps-3’s investigation of police corruption in the Phantom Solitaire case, linked to an interdimensional symbol. Earlier chronological appearances include a background cameo during Iwanome’s address to Comps-3 and a name mention alongside Tozawa.