Short for Exported Character, an Expy is a character who is quite similar to a character in another (or the same) series. Each character generally fulfills the same role and reacts to other people in the same way.
In Daria itself, The Daria Database shows us Andrea's webstrip "Gothic Nights", starring an expy of herself called Queen Hecuba; in each strip, Hecuba defeats an expy of someone from Lawndale that she really hates. Melody Powers is assumed in fanon to be Daria Morgendorffer's own expy. The urban legends in "Legends of the Mall" all feature expies of the Daria cast. Ken Edwards' story about an older man who has a relationship with a "developing woman-child" almost certainly stars an expy of his own creepy self.
An expy that becomes too much like a previously established character, to the point that only a change in name and outward appearance separate them, can be called a clone.
The expy, done badly, can actually become a Mary Sue character.