Ruijerd Superdia belongs to the Superd tribe, marked by emerald green hair, pale white skin, and a red forehead gemstone called the Third Eye. He bears a prominent facial scar and initially wears tribal attire, later adopting a headband to hide his gemstone and shaving his head for anonymity. A necklace from Rudeus Greyrat becomes a treasured possession.

Centuries ago, Ruijerd led the Superd warriors under Demon God Laplace. During the Laplace War, Laplace betrayed them, replacing their natural spears with cursed Devil's spears that induced a frenzy, causing indiscriminate slaughter, even among their own families. Under this curse, Ruijerd fatally wounded his son, Luiweld. Luiweld's destruction of the cursed spear freed Ruijerd, who, horrified, contemplated suicide but vowed revenge against Laplace. After years in hiding, he intervened in Laplace's battle against the Three Heroes, striking a decisive blow contributing to Laplace's defeat. Despite this, the Superds were forever branded monstrous killers, leading to their near extinction.

Ruijerd dedicated centuries to restoring the Superd tribe's honor, traveling alone to protect children and eliminate threats under a strict warrior code. His methods, however, often intensified fear of the Superds, earning him the moniker "Dead End." His isolation ended after the Fittoa Region Teleport Incident when he encountered stranded human children Rudeus Greyrat and Eris Boreas Greyrat on the Demon Continent. He pledged to return them to the Central Continent in exchange for their help rehabilitating the Superds' image. The trio formed the adventuring party "Dead End," performing heroic deeds to reshape public perception. Over their three-year journey, Ruijerd acted as a combat mentor to Eris, refining her aggression into disciplined swordsmanship. He clashed with Rudeus over moral approaches but grew to respect Rudeus's resolve to protect others.

After parting with Rudeus and Eris, Ruijerd continued searching for surviving Superds. Years later, he escorted Rudeus's sisters, Norn and Aisha Greyrat, to safety, offering Norn guidance on familial struggles. He later discovered Superd survivors in the Forest of No Return within the Biheiril Kingdom. This reunion was marred by a plague afflicting the tribe—Drain Syndrome—which Ruijerd concealed from the elders. He was secretly possessed by the Abyssal King Vita, who temporarily suppressed the disease; this possession manifested as a corrupted blue gemstone on his forehead. The illness was eventually cured by Cliff Grimoire.

Ruijerd adheres to an unwavering warrior code: "A warrior protects children and treasures their comrades." He exhibits fierce protectiveness toward the vulnerable, especially children, and becomes enraged when they are harmed. His traumatic past fuels a hatred for injustice, though he learns to temper his lethality after witnessing Rudeus's strategic compassion. Despite his stern demeanor, he forms deep bonds with those he trusts, displaying loyalty and paternal warmth. Survivor's guilt and the weight of his tribe's legacy drive his actions, though finding his people alleviates his solitude.

Ruijerd wields a trident-like spear forged from his deceased son's hardened tail—a Superd trait where tails naturally detach and ossify into weapons. This spear sharpens and hardens with use, becoming exceptionally durable over centuries. His combat prowess ranks among the world's elite, enabling him to battle Imperial-tier swordsmen and A-rank monsters. The Third Eye grants mana perception, functioning as a radar to detect life forces and hidden threats. He excels in stealth, evasion, and the spear-based martial art Sōjutsu. His Superd physiology grants immense physical strength, endurance, and a lifespan exceeding 500 years.