Jacob was an employee at Ernie York's electronic store.
What little is known about Jacob is gathered from Vinnie's Journal, a secret diary Vinnie stumbles upon in Vinnie's Tomb Chapter Two, written by Vinnie's Great Great Great Great Not So Great Grandfather (who was also named Vinnie).
Here we learn that the older Vinnie doesn't trust Jacob, and that he suspects Jacob planned to take over the store. Jacob was friends with "an untidy bunch of ruffians who wear clothes with white and black stripes" who installed some strange equipment in the store. This bunch of ruffians is presumably the local SZC (Sinister Zebra committee), who legends say were the ones who entombed Vinnie in the electronic store fifty-nine years ago, creating what is now known as Vinnie's Tomb .