The Duck is an entity created by engie_ninja during Destroy the Godmodder 2 to accompany TwinBuilder on his travels in the Twinmobile. The Duck has gained notoriety as a living meme within the community based off of the fact that it's a normal Aflac-esque duck adventuring with the literal Game Master of DTG2.
This meme-status is referenced by the fact that the Duck seemingly holds absurd power - it pulled a gun from out of nowhere during its first appearance, as shown in Ephemeralquest, and later picked a highly complicated lock within Project Binary's Moonbase. The Duck also seems to be sentient, as Ephemeralquest indicates it is capable of thought (yet not human speech), and it seems to want to keep its powers hidden from Twin.
The Duck is literally a carbon copy of the duck used in the Aflac insurance company's commercials. That's it.