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.
Names in other languages