Sōshi Minashiro originates from Tatsumiyajima Island, initially operating as the core system operator of the Siegfried System. He coordinates Fafner units against the Festum, a role causing emotional detachment and limited physical presence during early conflicts. He forms a critical pilot-system partnership with Kazuki Makabe, enabling effective Fafner deployment despite significant mental strain.

During "Dead Aggressor," Sōshi sacrifices himself by transferring his consciousness into a Festum to protect Tatsumiyajima, leaving behind a promise to return. His absence strains the psyches of those left behind, particularly Kazuki, who experiences physical deterioration while clinging to this promise. Sōshi later reappears through Misao Kurusu, a human-Festum hybrid claiming to carry his consciousness, facilitating his physical return to the island during "Heaven and Earth." His reappearance occurs amid heightened hostilities, positioning him as a pivotal figure in mediating the conflict due to his unique understanding of both humanity and the Festum.

In "Exodus" and "The Beyond," Sōshi resumes his role as a strategic commander while grappling with the psychological aftermath of assimilation. His experiences within the Festum collective provide critical intelligence about their motives and capabilities, informing Tatsumiyajima's defense strategies. He exhibits leadership during the Azure Operation against the Arctic Mir and coordinates efforts between human factions and sympathetic Festum.

Set after "Heaven and Earth" but before "Exodus," "Behind the Line" depicts Sōshi participating in a program allowing veteran pilots to explore civilian careers. This narrative focuses on his adjustment to daily life alongside fellow pilots Kazuki Makabe, Kenji Kondo, Sakura Kaname, Canon Memi, and Maya Toomi, examining their challenges transitioning from military to civilian roles after prolonged survival focus. His interactions reflect ongoing reconciliation between his human identity and residual Festum assimilation effects.

Throughout all media, Sōshi's development centers on bridging human and Festum perspectives, managing assimilation trauma, and fulfilling leadership responsibilities despite personal sacrifices. His relationship with Kazuki remains a consistent emotional anchor, influencing decisions in both combat and peacetime contexts. The character grapples with themes of coexistence, identity erosion through technology, and the psychological costs of perpetual warfare.