In one part of The Messenger there's a dragon which you have to ride. After this happens, the dragon starts talking to the protagonist. Because of its cartoonish appearance and the fact that it's a talking animal, Joe tried doing a Mickey Mouse impression for its voice.[1] Then the dragon introduced itself as Manfred, and it became a reocurring character throughout the game, and Joe kept on doing that voice for it. Now he calls his impression of Mickey Mouse the Manfred voice.

References
  1.     “Joseph Anderson Channel Two” video: “The Messenger - Surprisingly Great (Stream One)” at 04:09:51.