William J. Blazkowicz is a character from Wolfenstein series, and appears in Wolfentein RPG, and his descendants in Doom/Doom RPG series. Even had a crossover into Doom future in Mission III: Ultimate Challenge.
According to Tom Hall and John Romero several ID series are in fact connected. Doom Marine is a Blazkowicz descended from Commander Keen, and William J. Blazkowicz before him[1][2] (this does not apply to the MachineGames reboot universe however which follows its own universe with an alternate history for B.J. And different family tree). This is also mentioned in Doom RPG series as well. See ID multiverse timeline.
Background
William J. Blazkowicz was the son of polish immigrants, who served during WWII. He fought in many campaigns saved the world from the Nazis. In one campaign William was sent to the future, where he fought a demonic entity in a UAC facility.
In another campaign he fought the Harbinger of Doom who vowed to threaten his descendents (the creature would become the being known as the Cyberdemon).
Following the war he married an actress, their son was Arthur Kenneth Blaze, who became the father of William Joseph Blazkowicz II. His descendents both B.J. Blazkowicz III and Stan Blazkowicz saved earth and multiverse from incursions by demons from Hell. Blazkowicz III would be later known as the Doom Slayer.
Terror Billy
An alternate William J. Blazkowicz with a different family tree from a 'dark alternate reality' appears in MachineGames Wolfenstein series, and Quake Champions. In the latter he was brought from his particular universe in the multiverse to the Arena Eternal, and appears alongside Doom Slayer (who is supposed to be the same character through most of the Doom series) (B.J. Blazkowicz (Doom), son or grandson of Commander Keen). Note: Commander Keen's twin son and daughter in the reboot universe at least are also named Billy Blaze (William Blazkowicz).