Commander Keen is a character named Billy Blaze from a series of eponymous platform games by id Software. 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 destroy the four hanged Keens to trigger the door that hides the switch.
The Commander Keen can be considered a monster in the technical sense as it counts towards the kills percentage, will respawn under the corresponding settings, and is not included when the -nomonsters parameter is used. It does not, however, pose any threat to the player as the true monsters do.
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 Keen objects 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 got to take revenge against the cute character that he had to previously endure working with alongside its creator, Tom Hall, with whom Adrian is said to have had tensions during their time together at id Software. 
- 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 you kill them and the "raygun" sound plays when you damage them. The graphics used are a combination of original graphics and slightly modified Keen graphics from Commander Keen IV.
- Commander Keen (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 3d. Doomguy is implied to be a descendant of BJ in Wolfenstein RPG , and is later explicitly named as Stan Blazkowicz in its sequel, Doom II RPG. This series of connections makes the majority of id Software's protagonists related.
The IWADs contain the following numbers of hanged Commander Keens:
|Game||ITYTD and HNTR||HMP||UV and NM|
- Masters of Doom: How Two Guys Created an Empire and Transformed Pop Culture. David Kushner. Random House, LLC, 2004. pp. 65-66.
|Monsters from Doom and Doom II|
| Doom: Baron of Hell | Cacodemon | Cyberdemon | Demon | Imp | Lost soul | Shotgun guy | Spectre | Spiderdemon | Zombieman|
Doom 2: Arachnotron | Arch-vile | Commander Keen | Heavy weapon dude | Hell knight | Mancubus | Pain elemental | Revenant | Wolfenstein SS | Final boss