Commander Keen

Commander Keen is a character named Billy Blaze from 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 under the corresponding settings, and is not included when the  parameter is used. It does not, however, pose any threat to the player as the true monsters do.

Technical
Internally is used as a prefix for sprites relating to Commander Keen. The type of the object inside the source code is given the name. The sounds and  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.

Appearance statistics
The IWADs contain the following numbers of hanged Commander Keens per skill level:

Of the maps covered on the Doom Wiki, the following have the highest numbers of Commander Keens in single-player: This data was last verified on October 6, 2021.

Doom (2016)
Commander Keen is referenced at least three times in Doom (2016): a skull appears wearing his helmet (here emblazoned with the Doom logo) in Hell, one of the Doomguy collectibles is named "Keenguy" and appears with his outfit colors, and finally, a Mixom crate found in one of the game's multiplayer levels includes a picture of Keen apparently acting as the company's mascot.

Doom Eternal
Commander Keen's skull, helmet, and neural stunner are stored on top of a bookshelf in the Doom Slayer's library in the Fortress of Doom.

A shop in the ARC Complex level sells "Keen Candies" that include a picture of Commander Keen.