Baku, also called Bucky, is the son of King Bucky, ruler of Doedicos. His childhood ended when the antagonist Slash attacked, freezing the entire island. King Bucky remained trapped in ice, while Baku escaped with memory loss before being teleported to Primas, the first world. There, he grew into an anti-social college student who treated peers like slaves, though he held underlying care for friends beneath his cold exterior. His driving ambition focused on world conquest to become the supreme Great Child.

Spaak, the Great Child of Primas, witnessed Baku's courage and determination during a Trouble Monster incident and chose him as his successor. This granted Baku the spirit companion Jibac and launched his journey across the twelve worlds with allies Pinky and Kai. Throughout this quest, he confronted other Great Children influenced by Hail, a former Great Child of Doedicos, while aiding various worlds: In Tetras, he helped Dartagnan overcome familial trauma; in Pentas, he broke the curse afflicting Ali Babah; in Hexas and Novas, he mediated conflicts between siblings Live and Dead; in Seteras, he defeated the Toad Lord and challenged Runmaru's passivity; in Octas, he devised a plan for Lucy to reclaim her father's ship from bandits; in Dicas, he awakened as a Great Soldier to protect Hayate and purify a corrupted Holy Tree; in Undicos, he intervened in Joan's vengeance, purified her corrupted friend Ash, and learned the Purifier technique from Master Funen (disguised as Busta) to cleanse those afflicted by Slash's Poison of Awakening.

Arriving in Doedicos, Baku encountered his father's spirit, King Jibac, who revealed his lineage and Slash's past attack. King Jibac urged Baku to become Great Child of Doedicos, but Baku angrily refused, prioritizing his original dream of world conquest and showing minimal respect for his frozen father. Later, at Needle Tower, Baku confronted Slash possessing the Princess. With Spaak's assistance, Slash was expelled. When Slash resurfaced in dragon form, Baku destroyed him using the Princess's sword empowered by all twelve spirits.

This victory reunited Baku with fellow Great Children and led to the realization that his world conquest dream could not happen. Spaak's earlier gift of a Great Child license and watch represented the metaphorical fulfillment of his ambition, not literal rulership.

Personality-wise, Baku began cold and anti-social, often commanding friends like subordinates. His journey fostered greater compassion, seen in protective actions and willingness to help others. Confidence and intense focus defined his combat style against Trouble Monsters or when executing purification techniques.

Physically, Baku has long, spiky black hair with brown highlights and large black eyes, resembling his father. His primary attire is a red leather jacket, pale yellow undershirt, light purple shorts, white gloves, and blue-and-white sneakers. He adopted specific outfits: a black vest with a kimono and scarf in Seteras, and a military suit with a brown wig in Undicos.

Abilities include high combat concentration for targeting foes and notable durability, such as enduring Live's Soundwave Trap. His key techniques are the Purifier for cleansing corruption and the "Powerful Wave" for combat, amplified by his spirit companion Jibac.

Relationships include reliance on core companions Pinky and Kai, mentorship under Spaak and Funen, and adversarial dynamics with Hail and Slash. His bond with King Bucky remained conflicted, marked by resentment over past abandonment despite their familial tie.

Baku evolved from an isolated aspirant of domination to a leader valuing camaraderie and sacrifice. His understanding of "conquest" shifted from enslavement to protecting the twelve worlds, culminating in acceptance of his fulfilled dream after Slash's defeat.