...the girls aren't in love with Keima, but in love with the image Keima cold-bloodedly calculated would score the most affection points. For every single "conquest", rather than building a relationship between two people, Keima was using "I love you" to dig up and solve her problem.
Except Chihiro. She was in love with another guy, so Keima just got roped into that, not bothering to hide anything, because he didn't need to worry that he would contradict "the calculated image". Without him intending anything, she realized he had his cool moments.
Of all the girls, after finding out about this whole mess with the goddesses, and the escaped spirits, I think Chihiro's the only one that will have her image of Keima reinforced instead of distorted.