Kukuruku began as a wooden puppet cast aside by human owners after suffering mistreatment. This abandonment washed him ashore at the Ends of the Earth, a place gathering unwanted things. Centuries of sunlight there, perhaps two or three hundred years, mysteriously animated him, but his sun-baked existence also seeded profound resentment towards humanity.

He gained potent magical abilities: shape-shifting, flight often manifesting as a glowing bubble or streak of colored light, and the power to transform living beings into zombie-like "Living Puppets." These puppets became the raw materials for his fortress on Nightmare Island. His typical form featured a red robe, magician's hat, an elongated face with a pointed chin, and distinctive glowing red eyes. He could alter his body shape, frequently appearing spherical.

His driving motive was vengeance against humans for his abandonment. He recruited Toby as an apprentice, offering magical knowledge in exchange for Toby transforming others into Living Puppets and luring them to Nightmare Island. Kukuruku ruled through intimidation and manipulation, though Toby tried to protect his sister Cheri from this fate.

Confronted by Cheri and Unico, Kukuruku refused to undo his spells. During the battle, Unico perceived Kukuruku's deep loneliness and offered friendship. This gesture eroded the hatred sustaining Kukuruku's existence. An attack from Unico then critically wounded him, hastening his demise. With his hatred gone, Kukuruku reverted to an inanimate wooden puppet, which Cheri kept as a memento.

An early story concept by Osamu Tezuka presented him differently, as a possibly human prince named Kukurukku whose heart grew cold, leading him to pursue magic and torment others. His mother, Queen Urania, relayed this backstory before he departed to become a powerful wizard. This version did not appear in the final film.