Aubin is a character from Fire Emblem: Three Houses and the original bearer of the Crest of Aubin. He was one of the Four Apostles. After the Rite of Rising failed to revive the Goddess, he left Garreg Mach and eventually settled in an unknown town, presumably Abyss.
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It's strongly implied that Aubin was still alive until less than two decades before the plot began. Yuri Leclerc says that when he still lived with his mother in western Faerghus, they once took in an old but very wise and kind traveler. He first taught the young Yuri how to read and write, later saved Yuri's life during a fatal plague, and passed away nearly immediately afterwards due to natural causes.
Additionally, in Yuri's paired endings with male Byleth on all routes but Crimson Flower, it is stated that "informal notes and letters from friends suggested that the pair was extraordinary long-lived and their youth was untouched by the passage of time." This further cements the theory that the old man was Aubin, and that the cure he made for Yuri helped him live long and keep his youth.
Aubin is a masculine French given name variant of Albin, from the Roman cognate Albinus, derived from the Latin albus, meaning "white" or "bright".