Tsutomu Senkawa starts as an unremarkable high school student, his laziness offset by a fascination with abandoned structures—a hobby that culminates in a fatal encounter with an interplanetary agent hunting a fugitive. To survive, his consciousness merges into the agent’s body, binding them to a shared existence they must conceal. Plunged into fear and anxiety, he battles depression and academic decline as his reality fractures.

Gradually adapting, Tsutomu cultivates resilience and a drive for justice. Partnering with the agent, he employs his understanding of human behavior and historical patterns to aid investigations while juggling dual identities. Relationships with classmates, particularly childhood friend Natsumi Hayamiya—attuned to intimate details like scars and dental history—underscore his struggle to preserve normalcy. The reboot manga revisits his academic reliance on Hazumi and nascent bonds with peers Ryota Sudou and Kazuyoshi Chigira.

Physical transformations see him temporarily losing prescription glasses in certain timelines, his eyes shifting hue during the agent’s control. Empowered by the agent’s physiology, he gains heightened strength, speed, agility, and endurance, yet human frailty leaves him prone to collapse under extreme duress. In pivotal moments of anguish, he briefly reclaims autonomy, reverting to his original form.

His evolving personality blurs with the agent’s traits, edging toward a fused identity. The reboot delves into their symbiotic consciousness, where intertwined thoughts reshape decisions and emotions. As the narrative progresses, Tsutomu defies caution to mediate conflicts, fueled by a sharpened moral code. Shifting from bystander to active partner, he harmonizes human compassion with cosmic responsibility.

Scattered fragments of his pre-fusion life surface through Hazumi and Natsumi, the reboot subtly implying deeper ties with Hazumi. Natsumi’s keen observations become tools to authenticate his identity. Shared corporeal existence sparks chaotic family moments—misinterpreted as nudity or crossdressing—straining their clandestine arrangement.

Ongoing arcs probe existential threats of mental fusion, including irreversible assimilation. In the Decode timeline, Tsutomu’s resilience manifests as command over the agent’s form, marking his ascent into autonomy. His path ultimately bridges human fragility with otherworldly duty, balancing self-preservation and selfless resolve.