Very little is known about Jack.
He was born in England, hints have been dropped that it was in or around London.
He joined the Army during World War 2. His service record disappeared, so no one was sure what unit he was in or how he ended up behind enemy lines. (Dilios Note: Rumors and theories … both of which I hate speculating on … point towards him working as a spy – I’ve not yet had this confirmed and Shadow isn’t returning my phone calls)
Spike ran into him while he was fighting a group of Rogues, he was impressed that Jack was holding own against them. Once Jack finished off the last Rogue, with a flourish of his sword, he turned on Spike.
He still wasn’t sure how he ended up Turned and Mated.