Commander Keen

Commander Keen (aka "Billy Blaze") is a character from a series of platform games by id Software, named "Commander Keen" after him. 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.