In 'The Why of Fry' when Fry is finding out why the Nibblonians froze him, he finds out that Leela is worth saving in the Future. Nibbler refers to Leela as 'The Other'. What does that mean?
Nibbler does refer too Leela as "The Other" in EP "The Why of Fry" but we have yet too see what that future holds. Since we have not yet seen anything with "The Other" we just have too wait and see.
The exact nature of "the Other" remains unexplained, even with the events of the four DVD films and the first season of the second run. Speculation remains on the topic if the writers are going to address the nature of "The Other" in the future, but the line could also simply be of another reason. - The Other
So from all the facts we have so far we can't really know why Leela is The Other.
Although it is not exactly specified, "The Other" could be a reference to how she helped defeat the Big Brain in "The Day the Earth Stood Stupid." As said before, it has neither been confirmed or denied, but in the "Benders Big Score" David X Cohen said that most of the question were answered, so maybe that's all we'll get.