It looks like the flower that keeps appearing around Nagisa is called anemone coronaria.

It seems to be used as Nagisa's signature flower.

Mashiro has her own signature flower as well:

I just think that's neat. I wonder what Mashiro's flower is called.

I've decided to find out what Mashiro's flower is and I believe it is White Plumbago. I've looked it up, and apparently white plumbago is a common flower in Japan and it often means great hope and "holly wishes".