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Born to King Mors Lucis Caelum in M.E. 706, Regis Lucis Caelum CXIII spent his youth traversing Lucis with close companions Clarus Amicitia, Cor Leonis, Weskham Armaugh, and Cid Sophiar. These journeys forged him through battles against the Niflheim Empire, reckless driving resulting in multiple Regalia crashes, and mastering mechanical repairs. During this time, he encountered Ezma Auburnbrie, leader of the Hunters, pledging to combat daemonic threats in Lucian dungeons post-war. A later diplomatic mission to Accordo failed when Lucis lost crucial territories, compelling King Mors to contract the Wall solely around Insomnia, abandoning outlying regions to imperial aggression.

Regis ascended as the 113th King of Lucis following Mors's death in M.E. 729. He married his childhood friend Aulea in M.E. 732. Two years later, Insomnia faced assault during Founder's Day by the Adagium (later Ardyn Izunia) and Ifrit. Regis confronted the Adagium but was overpowered, narrowly saved by the intervention of the Founder King and Bahamut. Their son Noctis was born in M.E. 735 amid an emergency hospital dash that damaged the Regalia. Aulea died soon after, leaving Regis as Noctis's sole parent.

As king, Regis maintained the Wall—a magical barrier fueled by the Crystal and sustained through the Ring of the Lucii. This duty rapidly accelerated his aging, draining his life-force and leaving him frail, reliant on a cane. He recruited refugees from outlying territories into the Kingsglaive, granting them shared access to his magic for defending Insomnia. He also took in the orphaned Titus Drautos, who rose to become Kingsglaive commander.

A pivotal shift came in M.E. 741 when spirits of past kings revealed Noctis was the prophesied True King destined to eradicate the Starscourge. This intensified Regis's focus on securing his son's fleeting happiness. After a marilith daemon gravely injured Noctis in M.E. 744, Regis used Carbuncle to guide him from a coma. He took Noctis to Tenebrae for recovery, where the prince bonded with Princess Lunafreya Nox Fleuret. Niflheim attacked during this visit, forcing Regis to flee with Noctis while leaving Lunafreya and her brother Ravus behind—an abandonment Ravus never forgave.

Facing unsustainable magical depletion, Regis accepted Niflheim's deceptive peace treaty in M.E. 756, ceding all Lucian territories outside Insomnia and mandating Noctis's marriage to Lunafreya. Recognizing Ardyn's trap, Regis sent Noctis away with his retinue for safety, deliberately sacrificing himself to draw Niflheim's focus. During the treaty signing, Niflheim invaded. Regis fought alongside Clarus against General Glauca but was overwhelmed. Glauca severed the Ring of the Lucii from his hand, though it rejected Ravus. Escaping with Lunafreya and Kingsglaive soldier Nyx Ulric through a secret passage, Regis entrusted Lunafreya with the Ring for Noctis. He then sacrificed himself to hold off Glauca, declaring, "Locked doors will seal your fate no longer."

Regis's spirit joined the Lucii within the Ring. When Nyx later pleaded for power to save Insomnia, Regis advocated for him before skeptical past kings, securing their aid at the cost of Nyx's life. A decade later, his spirit manifested before Noctis in the Citadel during the final battle against Ardyn. Though visibly anguished when the Lucii attacked Noctis, Regis ultimately delivered the fatal blow alongside his ancestors to fulfill the prophecy.