Mutsumi Saburo, who goes by the name Saburo in the anime adaptation, is a human teenager attending the same school as Natsumi and Fuyuki Hinata. He carries a mysterious, laid-back aura and often skips class, preferring to spend his time on his own pursuits. Under the alias 623, he is secretly a popular radio DJ and an illustrator known for his quirky poems, making him a minor celebrity. His calm and observant nature allows him to remain unfazed in most situations, and he possesses a sharply intelligent and philosophical side. Saburo's primary motivation is his own curiosity and amusement; he engages with the bizarre events around him because he finds them interesting rather than out of a sense of duty. In the third movie, he plays a supporting role, using his resourcefulness and special abilities to assist the Keroro platoon during the high-stakes conflict. His most important relationship is with Sergeant Major Kururu, with whom he forms a close, mischievous partnership often called the "radio wave combo." Kururu gifted him a special pen that can bring anything drawn or written with it to life, a tool Saburo uses creatively in combat and problem-solving. He is also a figure of admiration for Natsumi, who harbors a crush on him, though he remains largely oblivious to her feelings, which creates a source of jealousy from Corporal Giroro. Saburo is a genius with an exceptionally high IQ and physical abilities far beyond average, making him adept at sports and piloting vehicles. Throughout the film, he remains a detached yet dependable ally, using his intelligence and the reality-altering pen to help turn the tide without ever losing his characteristic composure.