Inspector Koichi Zenigata, a Japanese Interpol officer originally from Tokyo's Metropolitan Police, dedicates his existence to capturing master thief Lupin III. This relentless pursuit drives him through extreme conditions, showcasing extraordinary endurance. Standing 181 cm (5'11") and weighing 73 kg, he bears a distinctive cleft chin and typically wears a trench coat—camel in Part 2, light green in Part III, red in Part 4, and light brown in Part 6. He carries standard handcuffs and a Colt M1911 pistol.

Zenigata combines fierce dedication to justice with emotional vulnerability, vehemently rejecting bribery yet displaying unexpected compassion, like temporarily caring for a child during the Statue of Liberty crisis. He weeps when Lupin shows kindness or appears dead. His law enforcement methods feature improvised physical feats during chases, where he unconsciously overcomes massive obstacles or subdues groups while fixated on Lupin. His signature move involves throwing handcuffs bola-style to restrain targets. He meticulously tracks Lupin's patterns, sometimes forcing the thief into retirement via police databases.

Across series iterations, Zenigata evolves from ruthless and violent in early portrayals—still holding grudging respect for Lupin—to a more compassionate, professional figure by Part 4. His dynamic with Lupin blends declared eternal enmity with protectiveness; he prevents others from capturing or killing the thief and occasionally allies with him against greater threats. Lupin's apparent death triggers profound grief, once leading Zenigata to retire and become a monk. His personal life remains overshadowed by his obsession, though a daughter named Toshiko is mentioned. When queried about family, he replies "not yet," implying his focus on Lupin precludes lasting relationships.