Amnael (real name: Lyman Banner) is a supporting character who is eventually revealed to be an antagonist, in the first season of Yu-Gi-Oh! GX. He is the seventh of the Shadow Riders, therefore the best duellist among them and the last to be introduced.
As a Teacher
In the beginning of the season Amnael is presented as Professor Lyman Banner, 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 theDuel Monster game and about Alchemy.
He appears as someone friendly, easy going and slightly goofy, who takes good care of his students, and he frequently gives advices and guidance to Jaden 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
As the first six Shadow Riders are defeated, Professor Banner seems to have gone missing. Shortly after, a mysterious masked duellist who introduces himself as the ancient alchemist Amnael appears on the Duel Academy island and starts challenging all the remaining keykeeper. (Duellists in charge of one of the six keys that keeps the Divine Beasts cards locked, since these cards wields a very dangerous mystical power that the Shadow Riders covet.)
Amnael beats Alexis Rhodes and Chazz Princeton easily, before taking Atticus Rhodes as a hostage. He then challenges the last keykeeper: Jaden. Jaden and his friends then follow Pharaoh into a cave where they discover an Alchemy laboratory and Professor Banner's mumified corpse in a coffin. Then Amnael appears and reveal that he trapped the souls of the duellists 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, rumoured 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 masks 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 Shadow Riders, hoping to use the Divine 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. (This happens only in the original japanese version. 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 duellist 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 Shadow Riders all by himself. Furhtermore, how the duels could have been rigged is not even explained.)
Jaden eventually realises 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 Shadow Rider to keep an eye on the others, and lied about the manipulation to force him to give his best.) In the grimoir, Jaden finds a powerful spell card "Sabatiel the Philosopher's Stone" which would later enable him to defeat the leader of the Shadow Riders Kagemaru.
Amnael 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.)