One of ten famous Alchemists and considered a world authority on catalytics – the study of making efficient alchemy - Wilhelm Eiselstein (ヴィルヘルム・エイゼルシュタイン Viruherumu Eizerusyutain ) was researching the “Philosopher’s Catalyst,” a legendary material that has powers comparable to the Philosopher’s Stone. When the town of Hiessgart (where he and his daughter resided) came under attack of chimeras, he brought the refugees to safety and led the efforts in building New Hiessgart.
Edward and Alphonse had met the professor and his daughter Selene sometime before events within Fullmetal Alchemist and the Broken Angel, while Edward and Alphonse had been studying with Izumi Curtis. Both brothers recall that Izumi regarded Eiselstein as a friend, and worry that he will tell their teacher about their attempt in Human Transmutation. The professor does guess that they have committed the taboo, but claims to have no interest in their personal lives, and so won't divulge this to Izumi.
To Edward and Alphonse's sorrow, they find Selene's grave in the former town, and meet a red-haired girl who Wilhelm says is his daughter, Armony. The mystery of Armony's relationship to Wilhelm is one of the big plots of the story of Fullmetal Alchemist and the Broken Angel.
He has continued to work on the Philosopher's Catalyst in secret, helping develop chimeras. A number of alchemists, who have run into trouble with the military, head to Hiessgart in the belief that the Professor is trying to create a society run by alchemists and independent of the Amestrian military. Whether Eiselstein actually took part in this, or turned a blind eye to it because of his hopes to create the Catalyst, is never made clear in the story.
Annoyingly, possibly due to the plot hole that video games are sometimes prone to develop, the names of the "ten famous Alchemists" aren't revealed, nor what determines their fame. As Eiselstein is not a State Alchemist, perhaps it is a reference to individuals of that caliber who choose to not enlist, in which case, Izumi Curtis would also qualify.
- Wilhelm likely takes inspiration from world famous physicist, Albert Einstein. His first name sharing a German origin much like Einstein and the similarity in family name being obvious.