Mikuru is a time traveler. She is assigned from the future to investigate the reason for the inability to travel to any point in time before three years prior to the present story.

She is very shy and timid and unfortunately, the odd end out of many a Haruhi eccentricity. Whenever she speaks of her mission or of her time plane, she inserts the word 'classified' into spots that would ordinarily reveal important information.
In the English version, she answers all of Kyon's probing questions with 'That's classified'.
Occasionally, Mikuru's older self will appear in the same time plane, essentially threatening a time paradox should anyone other than Kyon find out, meaning these visitations are exceedingly rare and unpredictable.

Source:Wikipedia