Professor Lyman Banner, known in the Japanese version as Daitokuji, is a teacher at Duel Academy in Yu-Gi-Oh! GX. He serves as the history teacher and the supervisor of the Slifer Red dormitory. On the surface, he appears to be a friendly, laid-back, and caring instructor, often considered weird or goofy by his students but still highly respected for his abilities. He has a well-known fondness for cats and is almost always accompanied by his large orange-brown cat named Pharaoh, whom he takes with him almost everywhere. He is also a gifted storyteller, especially when it comes to scary tales, and possesses broad knowledge that extends well beyond the subjects of history and dueling.

Beneath this cheerful exterior lies a far more complex history. Banner is a legendary alchemist who spent years traveling the world in search of the Philosopher's Stone. His research led him to discover an ancient tomb around the same period that Maximillion Pegasus first discovered Duel Monsters. He was eventually hired as an alchemy professor and head of the Slifer Red dormitory, but he was secretly conducting research under the orders of Kagemaru, the leader of the Shadow Riders. In this role, he lured several Obelisk Blue students, including Atticus Rhodes, into the abandoned dormitory, where they disappeared.

Banner's motivations are rooted in his failing physical body. After being stricken with a terrible illness, he created a homunculus in his own image and transferred his soul into it to save his life. When the homunculus began to fall apart, he sought the power of the Sacred Beasts in order to create the Philosopher's Stone and preserve his existence. This quest led him to become the seventh Shadow Rider under the alias Amnael.

Despite his direct responsibility for events meant to orchestrate the revival of the Sacred Beasts, Banner largely supports the protagonists for most of the first season. His true nature is revealed when he duels Jaden Yuki as Amnael, using an alchemy-themed deck, and his actions and intentions during this duel differ notably between the Japanese original and the English dub. After losing to Jaden, Banner gives him a shadow item and the card Philosopher's Stone - Sabatier. His original body and his homunculus then crumble to dust, and his soul comes to rest within his cat, Pharaoh, allowing him to remain connected to the events that follow.

In terms of abilities, Banner is a master of alchemy with enormous teaching talent and a deep understanding of many subjects. He was able to construct a homunculus and transfer his consciousness into it, demonstrating expertise that goes well beyond ordinary academics. As a duelist, he employs an alchemy-based strategy built around transmutation concepts, with Philosopher's Stone - Sabatier as his signature card. His relationships define much of his arc: he is a mentor figure to Jaden and his classmates, a subordinate and collaborator of Kagemaru, a figure responsible for Atticus Rhodes's disappearance, and in many ways the opposite of his colleague Dr. Vellian Crowler in temperament and demeanor, even down to their attitudes toward cats. His development traces a path from trusted mentor to hidden antagonist and finally to a reformed figure who aids Jaden and his friends in facing the threats posed by the Sacred Beasts.