I'm quite certain that this is Sakurai Rihoko from Amagami, not an original character. Can't access the Pixiv page now to double-check on anything, though.

EB said:
I'm quite certain that this is Sakurai Rihoko from Amagami, not an original character. Can't access the Pixiv page now to double-check on anything, though.

It was tagged original on the pixiv post, it does look similar to her though.