Commander Keen

Commander Keen (aka "Billy Blaze") is a character from a series of eponymous platform games by id Software. He can be found at the end of Doom II map 32 as an easter egg. Four Keens are hanging on ropes in the final room. You must kill all of them in order to be able to exit the level.

Technical
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.

Appearance statistics
The IWADs contain the following numbers of Commander Keens:

Trivia
The phrase in the prologue for MAP32: Grosse "You found the super-secret level. You better blaze through this one" is a reference to Billy Blaze.