Denjirō Tatsumi possesses grey hair, grey eyes, and a distinctive mustache. His typical outfit features a yellow shirt worn over a white undershirt, green pants, and often a white lab coat for professional settings. He exhibits a generally serious demeanor, though he becomes easily flustered by the antics of Hajime's parents and his own daughter, Kimari. His wife, Hanayo Tatsumi, observes strong similarities between Kimari's personality traits and his own. Denjirō is married to Hanayo, and they are parents to Kimari and Kouta. Professionally, he worked alongside Hajime's parents to develop the battle system central to the series' events.