Arnold Renck is a major character who serves as the Schwarz Knight within the Neo-Britannian Empire's Einberg order. Trained as a child soldier alongside Ash Phoenix, he frequently lost to him in sparring matches, fostering a one-sided rivalry as Ash did not share his competitive drive.

After Norland von Lunebelg's takeover of the Hokkaido block of Japan, Arnold joined him and was promoted into the Einberg, receiving his knightly title. He was later severely injured and nearly killed, then restored as a cyborg by the mad scientist Christoph Scissorman. This process utilized data from Bartley Asprius's research and studies on victims of Sakuya's Geass ability.

A key enhancement from this cybernetic restoration was the integration of a Geass Canceler into both of his artificial eyes, which appear normal when inactive. Unlike previous designs such as Jeremiah Gottwald's concealed device, this technology allows him to neutralize Geass powers.

Arnold possesses a highly competitive and vengeful personality, driven by a strong desire to defeat Ash Phoenix to settle their rivalry and avenge his fallen subordinates. He demonstrates dedication to his Einberg duties and loyalty to Commander Norland, while also harboring a personal hope to exterminate his enemies in one decisive encounter.

His story concludes during a sword fight with Ash Phoenix, where he is killed. The confrontation involved his advanced Knightmare Frame with automated defenses, which Ash's Zi-Apollo unit in Megistus Zeta mode ultimately broke through to defeat him.