I like the notion that it's on the same level as Xehanort, being X + No Heart or Another.
Xion - X + No I
fits with her having no idea who she is.
Well, like I said in that theory thread of Sigma's, I like this idea, especailly if it could relate to "not from single person," which would obviously cause the confusion that she has. ("Who am I?") Of course, she could just doesn't know by the time of that cutscene, but still. Anyways, like SA said, "No I" would fit well with the line, "Who am I?."