Ezra Parker was one of the financiers of the Vault 118 project. He was a business tycoon, owner of the Cliff's Edge Hotel and numerous other venture capital projects. Inside the Vault's residential wing, you will find a room filled with the various mementos that Ezra has collected from his projects and investments throughout the years.
Ezra Parker and all the other High Class residents were inside Vault 118 when the bombs fell. He could have been the one that initiated the override that kept the Overseer from bringing in the remaining residents.
Brain Dead Spoilers
Julianna Riggs made her way into the Vault 118 Overseer's Office. Inside she found the suspicions that the Overseer had of embezzlement against Ezra. She confronted him and he killed her before she exposed this scheme of his. He stayed around to get whatever more money he could get by taking on her personality.