John Malleus

 was the chief scientist of Vault 92.

He was in charge of implementing the experiment conducted by Vault-Tech on the residents of Vault 92 which was intended to subject some of the world's greatest musicians (who made up Vault 92's population) to white noise-containing subliminal messages. The hope was that this project could be used to create "super soldiers" who fought harder and with greater endurance for the United States military. Malleus was initially intrigued and optimistic about the results of the test, until those residents subjected to the white noise began to go violently insane. When the killings began, however, Malleus quickly got to the bottom of the situation and discovered that Rubin, the Overseer of Vault 92, had been brainwashing the musicians using the white noise without controls or safety measures, with "combat suggestions" that came from Vault-Tec itself, turning the residents of Vault 92 into violent, primal animals.

Malleus' conscience may have cost him his life, as Rubin's personal logs stated that the professor was an impediment to the project and was to be removed.

Appearances
This character is mentioned in Fallout 3 but does not actually appear in the game.

Behind the scenes
Malleus is quite probably named after one of the three auditory ossicles (the 3 smallest bones in the human body, located in the middle ear) which transmit sounds from the air to the inner ear. The malleus is the "hammer" in the set of "hammer, anvil, and stirrup," so named after their shapes.