Tsutomu Senkawa starts as an unassuming high school student whose passion for urban exploration leads to a fatal accident during a clash between alien fugitives and Federation agent Birdy Cephon Altera. To preserve his life, his consciousness merges with Birdy’s body, forcing an uneasy coexistence while his physical form undergoes reconstruction. Plunged into anxiety and depression, his academic performance falters and relationships strain under the weight of his turmoil. Pre-accident bonds surface through close friend Natsumi Hayamiya, whose intimate knowledge of his personal quirks—like a distinctive scar and dental history—later becomes key to confirming his identity in the reboot narrative.

Adapting to their shared existence, Tsutomu gradually masters control switches and tactical collaboration with Birdy. His personality transforms from a cautious student into a courageous ally, fueled by a burgeoning sense of justice. He complements Birdy’s missions with insights into human behavior and historical patterns, while she intermittently channels her superhuman capabilities through him. During a crisis, overwhelming emotional turmoil overrides Birdy’s form, temporarily reshaping her body into his likeness against her will.

The reboot delves into their merging psyches, hinting at irreversible fusion should their symbiosis persist. Mounting instability eventually pushes Birdy into a two-year stasis, granting Tsutomu sole command of their shared form. Secrecy complicates his civilian life: his father mistakes Birdy for him during a bath, while peers question his sanity as he converses with an unseen partner.

Tsutomu’s fractured family dynamic includes an absent mother, a sister sporadically monitoring his welfare, and a father fumbling through clumsy attempts at bonding. Though connected to friends Ryota Sudou and Kazuyoshi Chigira, his deepest ties remain with Natsumi—her sharp intuition relentlessly tests his constructed facade. Across continuities, Tsutomu evolves from a vulnerable bystander to a determined Earth defender, shaped by resilience and moral growth as he confronts extraterrestrial threats.