Lyman Banner, known in Japan as Daitokuji-sensei 「大徳寺先生」 is a teacher, a fabled alchemist, as well as a member of the Shadow Riders under the name Amnael, Banner largely supports the protagonists for most of the first season of the series, though he is directly responsible for numerous events meant to orchestrate the revival of the Sacred Beasts.
Banner's character design was overseen by Kenichi Hara. His usual outfit consists of a white shirt, black pants, belt and necktie. His black hair is tied back in a ponytail. Throughout the entire series, Banner's red eyes are constantly hidden behind their lids and a pair of square-lensed glasses, except during the second half of his Duel with Jaden.
In the dub, both Banner and his alter ego have Germanic accents (more obvious in Amnael's case) which is fitting, as many famous alchemists came from Germany.
The concept illustrations for Banner's facial expressions were identical for both Amnael and Banner.
His normal personality appears to be that of a laid back friendly teacher, often considered weird or goofy by his students. However his true personality shows him to be more focused and serious when it comes to personal matters.
Banner's personality varies greatly from his original Japanese counterpart in terms of his actions and intentions during the Duel with Jaden. In the English dub, Banner lies about having rigged all of Jaden's Duels so that he would be chosen as one of the Key Keepers and later on confesses that he joined the Shadow Riders to prepare him for the final battle against Kagemaru.
In the Japanese version, Banner never does this, but instead views Jaden as a "fusion genius" who must face adversity head on in order to learn the secrets of alchemy which have allowed him to triumph.
Banner was working on his alchemic research long before his arrival at Duel Academy, intent on creating the legendary Philosopher's Stone. His studies led him to discover an ancient tomb during the same period of time when Pegasus first discovered Duel Monsters.
Within, he unearthed three stone tablets adorned with the likenesses of the Sacred Beasts, and would later be stricken with a fatal illness. (In the English dub, he was cursed for this intrusion). To prolong his life, Banner developed a homunculus in his own image to transfer his soul into.
His new body however, began to crumble, so he resolved to joining the Shadow Riders as its 7th and final member in order to attain the power of the Sacred Beasts to fulfill his dreams of creating the Philosopher's Stone and use it to obtain immortality.
He would be present at the welcoming dinner for Jaden and all of the new Red students that year, introducing himself as the head Professor. Later, he would appear and scare Jaden and his friends when they were telling scary stories late one night, revealing the existence of the Abandoned Dorm, lying to them about being hired after the students went missing. He would later be the official judge of the Duel between Chumley and his father, determining whether or not he would stay at Duel Academy. Throughout the first year he would attempt to teach alchemy to his students, often with disastrous albeit comical results.
One night Banner is shown talking to Kagemaru on a computer screen, long after the Reds are asleep, arguing with him that Jaden might not be ready for the Gravekeepers test. The following day Banner goes through with this plan, and takes Jaden, Syrus, Chumley and Alexis as well as Pharaoh, who had invited himself hiding in Banner's bag, on a trip to the Gravekeeper Ruins.
Banner is shown in a cave at an unknown location with the other Shadow Riders, as Nightshroud voices he wants to make the first offensive. He is one of the seven Duelists that the Principal chooses to wield one of the spirit keys. After the arrival of Camula, he is shown in his room performing a traditional Japanese ritual to exorcise ghosts. Throughout the various battles with the Riders, he remains distant and chooses not to partake in any of the Duels, and eventually goes missing, but not before Atticus remembers that he was the one who led him to the Abandoned Dorm.
Using the pseudonym of Amnael 「アムナエル, Amunaeru」, Banner systematically defeats Atticus Rhodes, Alexis Rhodes, and Chazz Princeton taking their Spirit Keys to open the gate behind which the Sacred Beasts are sealed. He locks Alexis, Chazz and Atticus within his Shadow Charm, the Emerald Tablet and guides Jaden to his lair with his sigil to challenge him to a Duel. Before Amnael's challenge, Jaden finds his original body, and believes he's the reason that Banner has gone missing. At one point in the Duel, Pharaoh runs up and begins acting friendly towards "Amnael", causing Syrus to worry that he's a dangerous person. At this point Amnael takes off his mask and reveals his true identity, shocking Jaden and the others. Eventually despite his good strategy, Jaden is able to defeat Banner. Upon his defeat, Banner gives Jaden his Shadow Charm and with it, "Sabatiel - The Philosopher's Stone". His original mummified body and homunculus vessel then dissolve into dust. From then on, his soul resides within the body of his pet cat, Pharaoh, who had swallowed his soul.
In the second season, specifically during a Duel between Bastion and Jaden, Banner briefly escaped from Pharaoh. He happily exclaimed his freedom, before Pharaoh swallowed him again. When Jaden was constructing his Deck for his fifth Duel against Chazz Princeton, he led Jaden to the Ojama Trio in Reject Well in order to construct his Ojama-HERO Deck. He then returned to Pharaoh.
When Yubel-possessed Marcel Bonaparte caused the Spirit Gates to open up, Banner appeared again to inform Jaden what was happening. Marcel, who was angry at Banner for revealing his plan, caused him to disappear back into Pharaoh.
He then appears after Marcel starts running down the stairs to the Sacred Beasts, telling Jaden that the Sacred Beasts are very dangerous and that he must stop Marcel from obtaining them.
After the Yubel incident, he is still shown as a spirit, popping up out of Pharoah's mouth from time to time, talking with Jaden, with Jaden asking if hes ever going to the afterlife. He is also briefly captured and threatened with being flushed down the toilet by Jaden if he doesn't tell Jaden about what really happened in the Abandoned Dorm with him and Kagemaru, and about Nightshroud.
Then, after Jaden defeated Nightshroud and Jaden's match against Yugi Muto in the past, he (inside of Pharaoh) is discovered inside Jaden's backpack. He decided to stay with Jaden rather than stay in Duel Academy.
In Yu-Gi-Oh! GX Tag Force, Banner will not Duel the player unless the player currently has Pharaoh following him. He reveals himself as Amnael during the game's third part and will Duel the player and his partner alongside a brainwashed Jaden Yuki.
After completing the game once, Banner can be selected as a partner, though you must have Pharaoh following you when you ask him. If not partnered with the player, he will team with Ms. Fontaine during the tournament.
As Amnael, he appears in the game's third part and Duels the player and his partner alongside a brainwashed Jaden Yuki. After completing the game once, he can be selected as a partner should you defeat him in an individual Duel. He'll be found at the Abandoned Dorm after 10:00 PM in that instance.
As Amnael, Banner plays an Alchemy Deck, which draws a relationship between the science and universe. His strategy relies on the removal of his own cards from the game, supported by the Special Summoning of "Alchemy Beasts", based off common base metals and animals used in alchemy to transmute and refine gold, and the Alchemic Processes, based off four of the seven main stages in transmutation, falling under calcination, dissolution, separation, conjunction, fermentation, distillation and coagulation. In the Japanese version, the positions of the 2 words making up each Alchemic Beast's name are inversed, as well as "the" being omitted entirely. (Their names, as well as the names of the Alchemic Processes, are not hyphenated).
Lyman Banner runs a Deck focused on Fusion Summoning. This keeps in his theme of alchemy, as he says each time he Fusion Summons, "Alchemy teaches to combine elements into new and richer creations." (2 cards in Fusion Deck unconfirmed.)