Slaine Troyard, born on Earth as the son of Dr. Troyard, an Aldnoah technology researcher, relocated to Mars where he endured severe discrimination as an Earth-born citizen within the Vers Empire. His life transformed when Princess Asseylum Vers Allusia saved him during a near-fatal rocket landing, performing reverse mouth-to-mouth resuscitation to clear hyper-oxygenated fluid from his lungs. This forged a deep bond; Slaine became Asseylum's tutor, sharing knowledge of Earth that fueled her desire for peace. Their connection deepened after Asseylum comforted him following his father's death, further isolating Slaine in Vers society.

During the Second Earth-Mars War, Slaine served under Count Cruhteo but acted independently upon suspecting Asseylum might be alive. He tracked her to Earth, using Inaho Kaizuka's orange KG-6 Sleipnir as a lead. After aiding Inaho against a Martian Kataphrakt, Slaine was shot down by Inaho due to distrust. Recaptured and tortured by Cruhteo, Slaine refused to reveal the princess's survival. Count Saazbaum intervened, attacking Cruhteo's castle under the guise of rescuing Slaine but ultimately killing Cruhteo. Saazbaum revealed his role in orchestrating Asseylum's assassination, citing grievances against the Vers Empire's feudal system and his deceased fiancée. Sympathizing with Saazbaum's motives, Slaine aligned with him. He was later knighted for saving Saazbaum during an assault on Earth's headquarters, gaining the Tharsis Kataphrakt and a servant, Harklight.

Nineteen months later, Slaine had been adopted as Saazbaum's heir. However, harboring unresolved hatred for Saazbaum's attempt on Asseylum's life, he orchestrated Saazbaum's death in battle, using the Tharsis's predictive systems to time projectile strikes that crippled Saazbaum's Kataphrakt. Slaine then assumed the title of Count Slaine Saazbaum Troyard. He eliminated rivals like Count Marylcian in a duel, leveraging narrow tunnels to neutralize Marylcian's drone-based Kataphrakt before exposing him to vacuum. To consolidate power, Slaine accepted a marriage proposal from Princess Lemrina Vers Envers (disguised as Asseylum in propaganda), making him heir apparent to the Vers throne. He restructured Martian forces to prioritize collective conquest over individual territorial gains.

Slaine's descent accelerated when the real Asseylum awakened from her coma. To maintain control, he suppressed her memories and confined both her and Lemrina after Asseylum attempted to halt hostilities at gunpoint. His actions grew increasingly ruthless, disregarding Asseylum's peace directives and dismissing Earth's surrender offers. After Lemrina and Count Mazuurek exposed his deceptions, Asseylum regained her memories and allied with Lemrina against him. Slaine's forces abandoned him following public revelations of his treachery, leading to a final battle against Inaho. During their duel, Slaine expressed his hatred for Inaho's orange Kataphrakt before both crashed into Earth's atmosphere. Surviving, Slaine prompted Inaho to shoot him, but Inaho spared him at Asseylum's request. Slaine was imprisoned and blamed for instigating the war, though he was not responsible for the initial assassination plot.

Throughout his development, Slaine exhibited exceptional piloting skills, mastering complex aerial maneuvers and later the Tharsis with its predictive combat capabilities. His personality evolved from unwavering loyalty to Asseylum into a morally compromised ruler. Key traits included self-deprecation (often identifying as a "Terran" derogatorily), denial of Asseylum's disapproval of his methods, and an obsession with protecting her at any cost. His relationships were defined by devotion to Asseylum, strategic manipulation of Saazbaum and Lemrina, and rivalry with Inaho, whom he shot in a fit of protective rage. Despite initial ideals for peace, Slaine adopted Saazbaum's belief that Vers unity required Earth's subjugation, ultimately leading to his downfall.