Koichi Yamada, also known as Perman No. 5 or Parbou, is the youngest member of the superhero team. A two-year-old baby, he gains his Perman status after accidentally witnessing Mitsuo Suwa (Perman 1) using his powers. To preserve Mitsuo's secret, the alien Birdman recruits Koichi, equipping him with the standard Perman gear: a yellow helmet boosting physical abilities, a pink cape granting flight and super-speed, and a pink P-shaped badge enabling underwater breathing and communication with other Permans.

Despite his very young age and typical baby speech, Koichi demonstrates unexpected wisdom and competence. He actively engages in missions, combating criminals alongside the older Permans. His intelligence and tactical contributions sometimes surpass expectations for a toddler.

Koichi appears solely in the original manga serialization and the 1967 black-and-white anime adaptation. He is absent from the later 1983 color anime series and subsequent films. Within the manga, his appearances become infrequent over time, with nearly all chapters featuring him omitted from later compiled volumes. The reasons for his exclusion from later adaptations and manga collections remain unspecified.

His character arc shows minimal development beyond his initial recruitment and integration into the team. The sources do not elaborate on significant backstory, personal relationships, or future trajectory for Koichi Yamada.