Wilhelm Eiselstein is one of the ten most renowned alchemists in the world and basically the guy you go to if you want to understand catalytics—the art of making alchemy actually efficient. He's best known for his obsession with the "Philosopher's Catalyst," a legendary material supposedly as powerful as the Philosopher's Stone itself.
When chimeras attacked Hiessgart, the town where he lived with his daughter Selene, Eiselstein stepped up. He got the refugees to safety and spearheaded the whole effort to rebuild as New Hiessgart.
The Elric brothers knew him and Selene from their time training under Izumi Curtis, before the events of Fullmetal Alchemist and the Broken Angel went down. Both Ed and Al remember that Izumi considered Eiselstein a friend, which frankly made them nervous—they were worried he'd rat them out about their Human Transmutation attempt. But Eiselstein figured out what they'd done pretty quickly. He just told them he didn't care about their business and wouldn't say anything to Izumi.
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