Naofumi Iwatani started as a university student summoned to another world as one of four Cardinal Heroes defending against interdimensional Waves. Wielding the purely defensive Legendary Shield, he initially trusted openly. That trust shattered when Princess Malty Melromarc betrayed him, falsely accusing him of assault and stealing his gear. This vilified him across the kingdom, abandoned by fellow heroes into isolation, breeding intense bitterness and deep distrust. His survival hinged on pragmatic, morally gray decisions—including buying enslaved individuals. He purchased the young demi-human Raphtalia and later a filolial egg that hatched into Filo. Initially, he enforced loyalty through magical slave curses inflicting pain or threats of re-enslavement. Yet by providing medicine, training, and protection, he nurtured genuine trust. As Raphtalia and Filo grew into capable warriors, they formed a functional party, sparking his emotional recovery. Combat revolved around unparalleled defense and tactical support. His shield absorbed materials to unlock forms like Air Strike Shield for barriers and Shield Prison for containment. This defensive focus demanded ally reliance, honing his ingenuity. Rage fueled the Shield of Wrath, unleashing black-flame counterattacks at the cost of stat penalties and mental corruption. When disabled in another world, he gained the Vassal Mirror. He evolved from resentful isolation to reluctant leadership. Raphtalia anchored him emotionally and fought as his swordfighter; Filo offered mobility and combat aid. Queen Mirellia’s intervention cleared his name. Though he branded Malty and the king "Bitch" and "Trash" to humiliate them, he gradually shouldered duties beyond vengeance, prioritizing party protection and promises with guarded duty. Later, he engaged geopolitically: rebuilding Raphtalia’s razed village, Lurolona, using tournament winnings to free and resettle enslaved demi-humans. He confronted threats like the Spirit Tortoise, manipulated by antagonist Kyo. Bonding with its guardian spirit, Ost Horai, he faced desperation before her sacrifice steeled his resolve. Traveling to another world, he defeated Kyo, avenged Ost, and gained abilities like the Demon Dragon Shield. Relations with other Cardinal Heroes stayed strained. Queen Mirellia deemed him uniquely competent, but mutual distrust and perceived incompetence kept him apart from the Sword, Spear, and Bow Heroes. They coordinated minimally, even during Waves. Conversely, he allied with former foes—Glass and L’Arc Berg, Vassal Heroes from another world—transforming them into trusted comrades. Across all media, including Isekai Quartet, his core dynamic centers on fiercely protecting Raphtalia and Filo as family. He acts as their foster parent, preparing them for independence should he return home. Despite Raphtalia’s romantic feelings, he rejects intimacy, citing their caregiver-ward history to preserve their bond.