The Other
Turanga Leela, possibly "The Other".
"The Other" is a term used by Nibbler when Fry argues that Leela would be person worth saving in the future. Under the table at Applied Cryogenics, Nibbler offhandedly says, "she must be The Other", much to Fry's confusion (4ACV10).
The exact nature of "the Other" remains unexplained, even with the events of the four DVD films and the second run. In the commentary for "The Why of Fry", David X. Cohen implies that the story line with the Nibblonians goes beyond what we've already seen.
Other solutions
Since Nibbler states that Fry is the most important person in the Universe, he may have implied that Leela was the second most important person in the Universe.
One possibility may have something to do with the events of the second-run finale, "Meanwhile".
