Yuuma, a human boy mysteriously transported to the Fortress World, earns the designation "the Stranger." His arrival unfolds dramatically as his capsule descends into the Tower of Babel amidst a critical confrontation between the Fortress Knights and Dark Fortress. This sudden intervention inadvertently disrupts Dark Fortress's scheme to activate the Super Dimension Fortress Four destruction system.

He possesses the unique ability to fuse with the Fortress Knight Karot (Dragon Blue) via the Fortress Rise system, triggering a powerful transformation. This fusion drastically amplifies Karot's combat prowess, becoming vital to the Fortress Knights' struggle against Dark Fortress. Sustaining their combined power requires Yuuma to supply "strain energy" extracted from special fruits during conflicts.

On missions, Yuuma exhibits reactive responses to specific energy sources within the Fortress World. He displays a particularly strong reaction to volcanic energy during an infiltration of a volcanic island, resulting in an amplified transformation state termed "Super-Rise." Researchers Bulkan and Teurakan later provide him with a specialized weapon, the "Yuma Revolver," engineered to harness his energy compatibility.

Yuuma gradually adapts to his role alongside the Fortress Knights, participating in strategic operations across diverse environments like forests, underground bases, and volcanic regions. His presence helps rally other Fortress Knights and allies against Dark Fortress's tyranny, even as he initially contends with unfamiliarity regarding the Fortress World's technology and societal conflicts.