Leon Fayden commands the United States Navy's special warfare unit SWORD, leading covert operations to investigate suspected international robotics ban violations. He shares a close personal and professional bond with fellow SWORD agent Vexille Serra, a connection shaping their mission interactions.

When SWORD infiltrates Japan without authorization, security forces immediately resist. Leon faces capture early in the operation and is transported to Daiwa Heavy Industries headquarters. This capture strands Vexille as the sole free agent in Japan and casts doubt on Leon's survival, directly motivating later rescue attempts.

Imprisoned within Daiwa's facility, Leon witnesses firsthand Japan's concealed reality under corporate control. He observes the consequences of Daiwa's forbidden nanotechnology experiments: the forced transformation of Japanese citizens into synthetic beings. This confinement grants him direct insight into the societal collapse and environmental devastation wrought by rogue nanotech constructs called Jags.

A joint operation by Vexille and Maria, leader of the Japanese resistance, secures Leon's freedom. After rescue, he joins the resistance campaign to halt Daiwa's global expansion. During the climactic confrontation at Daiwa headquarters, Leon's visible concern for Vexille's safety provokes a jealous reaction from Maria, who harbors unreciprocated feelings for him. This emotional dynamic influences Maria's subsequent sacrificial actions against the primary antagonist, Kisaragi.

Following the destruction of Daiwa headquarters and the collapse of its operations, Leon is successfully extracted from Japan alongside Vexille. Their departure concludes his documented involvement in the events surrounding Japan's isolation.