Battsu, the Demon Lord Dragon, originated as a forbidden monster whose devastating rampage once engulfed the Dragon World continent. Sealed for centuries within an ancient shrine, this entity possesses an inherently tyrannical nature, defying all authority and resisting bonds with others. His imprisonment ends when Gao Mikado, Dragon World's representative in the World Buddy Masters tournament, inadvertently shatters the seal during a visit.

Despite Battsu's destructive history and initial hostility, Gao persists in forging an alliance, convinced friendship is possible even with such a formidable being. This leads to their unconventional partnership as they enter the World Buddy Masters tournament together. Their journey involves battling rival representatives from ten parallel realms and confronting an organization led by Wisdom, which seeks the legendary Mirage Card awarded to the tournament victor. Conflicts with Wisdom severely test their partnership, especially when Battsu faces dire peril or Gao's existence is threatened.

Throughout their battles, Battsu undergoes significant power transformations. He achieves a Thunder Emperor formation, demonstrating enhanced combat capabilities. Later, he unlocks an Overturn state, evolving into the Overturn Black Death Dragon, representing a surge in destructive potential. This progression culminates in mastering powers beyond Overturn, often unleashed in critical confrontations via techniques like the X-Tempest Ulti-Buster.

A defining aspect of Battsu's character is his gradual evolution from an isolated force of destruction towards forming a genuine bond. A key moment is the promise made with Gao under a starry sky, signifying a shift towards mutual trust. However, their partnership ultimately faces a profound challenge, culminating in a farewell that marks a significant emotional and narrative turning point.