I think girl loves bad "boy".
In this case, because Shizuru has soooooo many fans.
Everyone adores her... "onee-sama~" or "kaichou-sama" here and there lol.
While in the case of Natsuki, she has her own attitude.
She has this headstrong character even if its towards Shizuru.
Perhaps Shizuru falls for this straightforward, head-strong character of Natsuki.
Like the feeling of
everyone is nice to me, yet she is really different; she dares to show me attitude. (minus Haruka lol)
Not to mention that Natsuki is really hawt & cool on the outside, but in fact is a really shy & gentle girl.
Natsuki is like a treasure chest, the more u get to know her, the more interesting stuffs u will find out. E.g. she is a bra maniac, blushes easily... All are so different from what her appearance shows.
Hehe...so Shizuru must have loved the excitement of unvealing treasures...
*hoping for mooooooooore ShizNat action in OVA*