Cruz Schild is a central figure in the narrative, a young boy who finds himself thrust into a violent world for which he is initially unprepared. He is the sole survivor of a failed rebellion against the powerful Simeon organization, an event that shapes his early experiences and motivations. Deeply attached to his older sister, Aruka, he believes she died in the attack and carries a pendant with her picture, often speaking to her in his thoughts. This emotional anchor complicates his journey, as he later faces the devastating truth that Aruka not only works for Simeon but was also ordered to kill him to retrieve a hidden data chip.

At the outset, Cruz is portrayed as physically weak and prone to despair, struggling with feelings of helplessness. Lacking supernatural abilities, he is often treated as an errand-runner by his more powerful allies. However, his profound weakness is balanced by an exceptional intellect. Cruz possesses remarkable deductive reasoning and observational skills, which he uses to analyze enemy abilities and devise strategies that prove vital to his team's survival. His insights allow others to identify critical weaknesses in opponents' techniques, cementing his role as the group's strategic thinker when direct combat is impossible for him.

His personality evolves significantly through constant trauma and failure. Initially a boy who easily gives up, he learns to contribute in the only way he can, encouraged by allies like Eve Neuschwanstein to use his mind when he cannot fight. A significant and unconventional part of his development involves cross-dressing as a girl named Yamada, or Yamada Nadeshiko, to evade Simeon forces within the Killer Girl Squad Academy. What begins as a desperate disguise becomes a prolonged element of his character, as his friends insist he maintain the persona for safety and their own amusement, forcing him into uncomfortable and comedic situations.

Key relationships define Cruz's growth. His bond with his sister Aruka is the most painful, as he must reconcile his love for her with her betrayal. Within his group, his interactions with the carefree and kind Mio bolster his self-worth during periods of isolation, helping him find confidence. His relationship with the powerful Needless Blade shifts over time; Blade and Eve initially see him as a gopher, but through his unwavering courage and strategic value, Cruz earns their genuine respect, with Blade finally acknowledging him by his true name.

Throughout most of the story, Cruz is believed to be a normal human without a Fragment, the term for a supernatural ability in this world. Yet, his uncanny survival in situations that should have killed him are later revealed to be hints of his latent power. After a series of traumatic events, including witnessing his sister's death and being stabbed by the primary antagonist, Adam Arclight, Cruz finally awakens his own Fragment: the Shield of Aegis. This Stigmata-level ability generates powerful anti-matter shields from his palms capable of nullifying virtually any attack. This long-delayed manifestation represents the culmination of his journey from a helpless, "needless" boy to a being of immense latent power who has earned his place among the super-powered fighters.