Arnold Renck serves as the "Schwarz Knight," or Black Knight, within the Neo Britannian Empire's elite Einberg order. A loyal and formidable warrior, he is a proud and fearless knight devoted to his commander, Noland von Lüneberg, acting as his right-hand strategist and following his orders with unwavering dedication. Driven by a fierce sense of duty, he shows no mercy toward anyone who opposes Neo Britannia. At his core, he is defined by an extreme, one-sided rivalry with Ash Phoenix, a former comrade from his past as a child soldier. While Arnold is obsessed with defeating Ash to settle their history and prove his superiority, Ash never truly shares this competitive drive, which only deepens Arnold's frustration and animosity.

In his initial appearance, Arnold Renck is a handsome young man with striking blue hair and bright orange eyes, often seen with a confident, almost arrogant smirk. As a pilot, he relies heavily on the overwhelming defense of his custom Knightmare Frame, the Valpnir. This machine is equipped with automated shield units that detect and block enemy attacks from all angles, a fighting style that underscores his pride but also his dependence on technology. His first major confrontation occurs when he is sent to protect the Abashiri Detention Center. Anticipating an attack from the resistance group known as the Seven Shining Stars, he leads his forces into battle. During the fight, he recognizes his opponent as his old rival, Ash Phoenix, and eagerly engages him. Confident in his machine’s impenetrable defenses, Arnold is shocked when Ash criticizes his over-reliance on his equipment and proceeds to defeat him, piercing the Valpnir’s cockpit. Although he appears to die in the ensuing explosion, Arnold miraculously survives, though he is left on the brink of death.

Rescued and reconstructed by the scientist Christoph Scissorman using advanced imperial cybernetic technology, Arnold undergoes a radical transformation. His body is enhanced with cybernetic implants, and he receives a Geass canceller in both of his artificial eyes, a powerful device that can nullify the effects of Geass, similar to the technology once used by Jeremiah Gottwald. This resurrection changes him profoundly. His hair turns an unnatural white, and his appearance becomes far more sinister, mirroring his deteriorating mental state. His confidence warps into a megalomaniacal arrogance, and he loses much of his grip on reality, proudly declaring that he has become the ultimate fighting tool himself, thus dismissing any criticism about relying on machines. With his new, advanced Knightmare Frame, the Lowengrin, which syncs directly with his enhanced senses and reflexes, he returns as a far more dangerous and unhinged adversary.

Upon his return, Arnold seeks to crush Ash and the resistance. Using his Geass canceller, he is able to temporarily free Ash from the Geass binding him, leaving him vulnerable, and nearly kills him before being stopped. In the final act of the story, Arnold’s obsession consumes him entirely, and he cares about nothing other than finally defeating Ash in a deadly duel. However, even with his incredible new power, he is ultimately outmatched and meets his end when Ash throws him onto a sharp piece of rubble. Despite all his efforts and suffering, Arnold is never able to best his rival, ending his story as a tragic figure. He remains a tool used by Noland and a man whose pride and vengeance lead to his ruin, never receiving the acknowledgment from Ash that he so desperately craved. His key relationships are defined by his blind loyalty to Noland von Lüneberg and his obsessive hatred for Ash Phoenix, though he is shown to be on friendly enough terms with fellow Einberg knight Catherine Savasula, who calls him by the nickname "Ar-kun".