He was voiced by Wayne Grayson.
As a Teacher
In the beginning of the season Amnael is presented as Lyman Banner (Daitokuji in the Japanese dub), the teacher in charge of the Osiris Red dorm, where the least skilled duelist in the academy (at least in term of grades) reside. He gives lessons about the origins of the Duel Monster game and about alchemy.
He appears as someone friendly, easygoing, and slightly goofy, who takes good care of his students, and he frequently gives advice and guidance to Judai and his friends, witnessing almost all their duels throughout the season, and never being seen far from his pet cat Pharaoh.
As a Shadow Rider/Seven Stars Assassin
As the first six Seven Stars Assassins are defeated, Lyman Banner seems to have gone missing. Shortly after, a mysterious masked duelist who introduces himself as the ancient alchemist Amnael appears on the Duel Academia island and starts challenging all the remaining keykeeper (duelists in charge of one of the six keys that keeps the Phantom God Beasts cards locked, since these cards wields a very dangerous mystical power that the Assassins covet).
Amnael beats Alexis and Chazz easily, before taking Atticus as a hostage. He then challenges the last keykeeper: Jaden Yuki. Jaden and his friends then follow Pharaoh into a cave where they discover an alchemy laboratory and Banner's mummified corpse in a coffin. Then Amnael appears and reveal that he trapped the souls of the duelists he defeated within his grimoir, the Emerald Tablet, in which he concentrates his alchemic power. (Amnael's book is named after a legendary book of alchemy, rumored to have belonged to the fabled alchemist Hermes Trismegistus.)
Amnael proves himself to be a formidable foe, but Jaden manages to hold his own. Amnael's true identity is revealed when his mask falls apart. Amnael explains that he fell terminally ill during an experiment, forcing him to transfer his soul into a homunculus who looks exactly like him. However, the homunculus has a very short lifespan and Amnael joined the Seven Stars Asassins, hoping to use the Phantom God Beasts' power to create the Philosopher's Stone and become immortal.
Amnael then gains the upper hand and starts telling Jaden that they are rather similar and that the way he duels is somehow close to alchemic principles. In the "4Kids" dub, he makes a rather classical villainous speech about how Jaden was manipulated, so that he would win all his duels and look like a powerful duelist and be entrusted one of the keys, while he is in fact a weakling. It must be noted that this speech is hardy coherent, since Jaden defeated most of the Seven Stars Assassins all by himself. Furthermore, how the duels could have been rigged is not even explained, though it would later be revealed that he lied about all that.
Jaden eventually realizes the truth in Amnael's words and wins the duel by fusing four monsters of the Earth, Water, Wind and Fire elements (uniting the elements being a major theme in Alchemy). Amnael then gives him the Emerald Tablet and appoints him as his successor, before crumbling into dust. (In the "4Kids" dub, he tells him that he became a Seven Stars Assassin to keep an eye on the others, and lied about the manipulation to force him to give his best, in order to see if he was ready for the true threat.) In the grimoir, Judai finds a powerful spell card "Philosopher's Stone - Sabatier" which would later enable him to defeat the leader of the Seven Stars Assassins, Kagemaru.
Amnael's spirit survives, but gets eaten by his pet cat Pharaoh. During the rest of the series, the spirit leaves the cat's mouth from time to time and gives advice to Jaden.
Amnael wields an "alchemy" deck which focuses on alchemy principles such as creating living chimeras and homunculi, refining matter to turn it into something purer and making the body evolve to become mightier.
His deck focuses on field cards that remove cards from the field to empower his monsters. He also uses magic cards that turn into chimera-monsters able to strike his opponent's life-points directly. His signature monsters are the Helios Megistus series: three monsters (Helios Megistus, Helios Duo Megistus and Helios Tris Megistus, whose name is reminiscent of Hermes Trismegistus. When one of them is destroyed, it automatically summons the following one who is more powerful, akin to the alchemic concept of evolution).