Inspecteur Yasui is an elderly Tokyo police officer with a slightly stocky build, short choppy gray hair, and prominent facial wrinkles. He typically wears a white dress shirt under a formal pale brown blazer. His demeanor is laid-back and friendly, a contrast to his partner Takai. Yasui approaches investigations with calm experience, occasionally displaying kindness, such as acknowledging Keisuke Suga's emotional state during an interrogation. His methods include practical steps like suggesting online searches for clues. Yasui's involvement focuses on pursuing Hodaka Morishima. He first appears chasing a host linked to Hina Amano's harassment, later scanning CCTV footage of Hodaka wielding a gun and interrogating the host about the boy. Over ramen with Takai, they conclude Hodaka is likely a runaway. Yasui then visits Keisuke Suga's residence to inquire about Hodaka, observing Keisuke's reaction during the conversation. At the Yoyogi Kaikan climax, Yasui attempts to intercept Hodaka fleeing police custody. While Takai is restrained by Keisuke, Nagi Amano tackles Yasui, allowing Hodaka to escape onto an emergency staircase. His name incorporates the kanji for "safe" (安) and "well" (井), sharing the final kanji with Takai.