From the Super Mario Wiki
D-Men are goblin-like creatures that work for Queen Jaydes that reside in The Underwhere in Super Paper Mario. There are two types of D-Men. First, there are D-Men that work directly with Queen Jaydes, who wear suits and are flooded with work. The second type of D-Men wear loincloths and are mostly seen in the Underwhere Road and make sure people safely climb upward to The Overthere.
The D-Man's name is a pun on "demon" and "G-Man", which is short for "Government Man" and is a slang term for FBI Agents; this refers to the fact that they are agents of Queen Jaydes, the role played by demons in the real world mythology that The Underwhere is heavily based upon.
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