The girl he likes is not an obvious choice since Asuna says "Why didn't I think of that?"
That should eliminate: Nodoka, Yue, Asuna, Anya, Ayaka, Ku, Cheerleaders, Ako
Possibly eliminates: Chisame, Makie
The second hint from Asuna is "But can you two actually hook up? I mean, you two are kind of the same people." So these girls still stand a chance.
Konoka/Setsuna: Either one would be an obstacle for the other. Everyone already wrote them as a yuri couple but they could be very close BFFs.
Yuna: Both are obsessed with their fathers. Yuna's papa love would be an obstacle.
Eva: Both are ME masters. Negi would encounter the Nagi obstacle.
Other possibilities: Akira, Chizuru