Tsukasa Futaba, alias "Soul," wields the power to transform into a Wave-Human via symbiosis with FM-ians—electromagnetic wave entities from beyond Earth. Orphaned early, they were nurtured by a grandfather steeped in FM-wave research, cultivating a deep understanding of wave dynamics that later becomes pivotal in clashes between humans and FM-ians.

Initially ensnared by the FM-ian King’s influence, Tsukasa emerges as an antagonist under the "Soul" mantle, clashing with the protagonist. Yet even amid manipulation, flickers of loyalty and compassion surface. Upon shattering the King’s control, Tsukasa forges an alliance with the protagonist, bonding over shared Wave-Human experiences and mutual resolve.

Their Wave-Human form grants fusion with FM-ians, amplifying physical and combat prowess. Twin Leaders—bladed tonfa weapons—channel their agility in battle. Outwardly cheerful and supportive, Tsukasa harbors doubts about their hybrid existence, a tension dissected through alliances and rivalries. Bonds with peers, including a steadfast childhood friend, weave narratives of trust, culminating in Tsukasa reframing duality as empowerment.

In later conflicts against the FM-ian King’s remnants and emerging threats, Tsukasa leverages strategic acumen and combat skill. Their grandfather’s studies remain a cornerstone, offering tech-driven solutions to wave-based crises. Mediating disputes between rogue FM-ians and human-wave hybrids, Tsukasa bridges divides, advocating coexistence.

The arc concludes with Tsukasa vowing to safeguard both species, embodying unity between humanity and FM-ians. Resolving identity conflicts and nurturing unshakable camaraderie, they embrace their dual heritage, cementing their legacy as a bridge between worlds.