[Kogetsu Shirogane]

I honestly expect that, if she really wanted to, Haruhi could probably make it so Mikuru ends up failing miserably and repeating the third year(it was actually stated as a possibility, albeit for second year)... of course, unless Haruhi realizes her true potential by then, she'll probably just pass it off as a rather beneficial coincidence.

And I'm still against the "slider girl"=Kyon's Sister thing, if not for what I mused about earlier, then only due to the fact that she'd also have to be temporally displaced... if anything, it's just a big red herring thrown in simply to confuse the hell out of people.