According to Tom Hall and John Romero several ID series are in fact connected. Doom Marine (in Doom/Ultimate Doom) is a Blazkowicz descended from Commander Keen, and William J. Blazkowicz before himThis is also mentioned in Doom RPG series as well. See ID multiverse timeline.
This may not apply to the MachineGames reboot universe however which follows its own universe with an alternate history for B.J. and different family tree, though Billy and the Blaze Twins exist in that universe as the son and grandchildren of one of B.J's daughters (or a future son). We have not yet seen the Slayer's counterpart from MachineGames universe.
William J. Blazkowicz II (Billy Blaze) is the grandson of the great W.W.II hero William J. Blazkowicz (B.J. Blazkowicz).
Even though Keen is only eight years old, he's highly intelligent for his age; it's said that he has a IQ of 314. Commander Keen usually wears a purple t-shirt, jeans, and an American football helmet painted in Green Bay Packers colors that he borrowed from his brother.
He took his name from one of his grandfather's buddies from WWII, who was named Commander Keen.
An incarnation of the character can be found in the super-secret Doom II map, Level 32: Grosse, as an easter egg. In the final room of that level, four hanged Commander Keen bodies are suspended from ropes by their necks around a metal cube containing the exit switch. The player must kill the four hanged Keens to trigger the door that hides the switch.
When killed, a Commander Keen emits an electronically-distorted death cry as he falls off his hanging rope to the ground, his eyes wide open facing the player with a horrified look, middle body torn apart showing bloodied organs and ribcages, but his head and blood-soaked tennis shoes are clearly left intact. The horrified face Keen displays when killed is the one the player sees on Billy when he flies out of the level upon losing a life in the original Commander Keen games.
Commander Keen can be considered a monster in the technical sense as he counts towards the kills percentage, will respawn under the corresponding settings, and is not included when the -nomonsters parameter is used. He does not, however, pose any threat to the player as true monsters do, meaning he cannot attack the player.
Slayer's Club describes them as hanging effigies of the previous id hero Commander Keen.
Commander Keen's skull on a pike can be found in Doom. Some believe this to be a double reference to both Romero on a stake in Doom II, and Commander Keen. The Keen Skull is like a reference to the four Doom II Commander Keens. When their grandson kills them, only their head is left behind.
The Keen Skull becomes a reoccuring theme in later games in the Doom 2016 series.
A hologram in Doom VR includes Keen as a skull in the classic football helmet.
A record by Commander Keen called Shadows Don't Scare can be found in Doom Slayer's room. Again the record shows Skull Keen emblem again.
He is also the mascot of Keen Candies, which appear to be Chew Gummy Daisy Rabbits...
Some believe this is another confirmation of a 'shared universe'.
Internally "KEEN" is used as a prefix for sprites relating to Commander Keen. The type of the object inside the source code is given the name "MT_KEEN". The sounds "DSKEENPN" and "DSKEENDT" are used when the Keens are injured and killed, respectively. When all the Keens in a map are killed, any sectors tagged 666 will open as doors.
- According to John Romero, the hanged Commander Keens were Adrian Carmack's idea and execution. Preferring to work on dark and gory themes, he had his revenge against the cute character he had to "endure" working with previously.
- Tom Hall notes that Keen's grandson killed four of his grandfathers. According to him, in his story, that was okay.
- The phrase in the prologue for Level 32: Grosse contains a reference to Keen's name, Billy Blaze: Congratulations, you've found the super-secret level! You'd better blaze through this one!
- The sounds used for the Keens are taken from the original Commander Keen games in that the Commander Keen "level start" sound plays when the player kills them and the "raygun" sound plays when the player damage them. The graphics used are a combination of original graphics and slightly modified Keen graphics from Commander Keen IV.
- Billy Blaze (legal name: William Joseph Blazkowicz II) is the grandson of Wolfenstein 3D's William Joseph "BJ" Blazkowicz according to The Official Hint Manual for Wolfenstein 3-D. The Doomguy is also implied to be a descendant of BJ in Wolfenstein RPG. It was later confirmed by John Romero and Tom Hall that Doomguy was the son of Commander Keen and thus the great-grandson of BJ Blazkowicz.
- In an early version of Brutal Doom, Commander Keen's head is replaced with that of pop singer Justin Bieber. When killed, the head just falls to the ground without any effects, although they still use the original sounds when injured or killed. Keen was changed back to normal as of v0.19.