Amnuel is a significant character from the first season of Yu-Gi-Oh! GX, and his true identity is that of Lyman Banner, the seemingly eccentric and good-natured professor in charge of the Slifer Red dormitory. The name Amnuel is a pseudonym Banner adopts when he reveals himself as the seventh and final member of the Shadow Riders, a group of duellists working to resurrect the powerful Sacred Beasts. His background is deeply rooted in the study of alchemy. Before becoming a teacher at Duel Academy, Banner was a dedicated alchemist searching for the legendary Philosopher's Stone. His quest led him to discover ancient ruins containing the stone tablets of the Sacred Beasts, an event that cursed him or left him with a fatal, deteriorating illness. To cheat death, Banner used his alchemical expertise to create a homunculus, an artificial body, into which he transferred his soul. However, this new vessel was also temporary and began to crumble, which ultimately drove him to join the Shadow Riders in hopes of harnessing the power of the Sacred Beasts to achieve true immortality and complete his alchemical research.

In his daily life as Professor Banner, his personality is that of a laid-back, friendly, and somewhat goofy teacher who is genuinely kind to his students in the Slifer Red dorm, particularly Jaden Yuki and his friends. He often uses his alchemy lessons for comedic effect and enjoys telling scary stories, notably the legend of the Abandoned Dorm. This persona contrasts sharply with his identity as Amnuel. As Amnuel, he becomes a focused, strategic, and formidable duellist who speaks in riddles about alchemical principles. His primary motivation is survival and the completion of his life's work, but his role in the story reveals a deeper purpose. After being chosen as one of the seven Key Keepers to protect the Spirit Keys, Banner eventually goes missing. As Amnuel, he systematically defeats other Key Keepers like Alexis Rhodes and Chazz Princeton, stealing their keys and trapping their souls, along with that of Atticus Rhodes, within his Emerald Tablet Shadow Charm. This sets up a final confrontation with Jaden Yuki, whom he sees as his greatest pupil. During their duel, Amnuel pushes Jaden to his limits, but it is ultimately revealed that his defeat is not for the sake of the Shadow Riders' leader, Kagemaru. Instead, Banner's true intention was to test Jaden and prepare him for the greater battles to come, gifting him his Shadow Charm which contains the powerful Philosopher's Stone - Sabatiel.

Key relationships define his character. His bond with his pet cat, Pharaoh, is particularly important, as the cat is a constant companion. Furthermore, after his physical body disintegrates upon his defeat, his soul is swallowed and continues to reside within Pharaoh, allowing his spirit to remain present in the series. His relationship with Jaden is central; he moves from a mentor figure to an antagonist, and finally to a guide who entrusts Jaden with the tools to save the academy. As a duellist, Amnuel is notable for his unique Alchemy Deck, which directly reflects his life's work. His strategy revolves around removing his own cards from play to special summon "Alchemy Beasts" and activate "Alchemic Processes". This buildup allows him to summon powerful monsters like the Golden Homunculus, whose strength grows with every card removed from play, and the Helios series of monsters, which evolve when destroyed. This meticulous, process-oriented style of duelling is a direct representation of alchemical transmutation and makes him one of the most strategically complex opponents in the first season.