I don't know the full story but at one point in the vault's history there was a rebellion. The overseer was either killed or overthrown, I'm not sure which. Anyway, the leaders of the revolution didn't want to entrust the fate of the people to another overseer so they installed this mainframe. The records say it was purchased from... oh, what was that name... Ah, I remember, the 'Brotherhood of Steel.
Jimmy is a tall, lanky man who wears glasses. He is protective of the equipment in the vault, cautioning the player not to touch it. Jimmy's specialization is software, not hardware, making him unable to fix the malfunctioning vault computer.