The following were notes while the character was evolving. They do not necessarily represent the way it came out.
Agatha's mother, and the source of much mystery. Rumor has it that Von Pinn was in some sense built from her. We know that the Other is, in some sense, also her. Definitely not in-game -- she's too central to the main story, and too little understood for me to make guesses about her details. That said, need to think carefully about how she relates to Von Pinn, and how she relates to Helen. (Presumably Helen's mother is a dead ringer for her.)
The story of the Geisterdamen, told on (v5, p80), is also clearly a key page in the story of Lucrezia. One gets the impression that it was she who originally came to and ruled the Geisters, but the Other who came back later and exiled them.
Agatha finds out that her mother was the Other on (v5, p87). She immediately twigs to the inconsistencies that presents, which I believe supports the theory that there is some difference between the two. And Klaus observes that [the Other's technology is like Lucrezia's, but much more advanced]. And Agatha notes that [Lucrezia's device is far beyond her]. Could the Other be a time-traveling descendent or some such?
On (v5, p97), she remarks that it has been "so long" since she was "really human". Combine that with the image of her armored hand we see in the Geisterdamen history, and one gets the distinct impression that she went through a period of being a Clank or some such.
Note that Klaus thinks that the Other [could be Agatha], not necessarily Lucrezia. For all we know, this is yet possible.
Amount of Character Potential: Out (I don't want to deal with this particular complication: it's too tied into unknowns that are currently being explored.)