The Natsuiro Kiseki official site has some good pics of the four main characters and their seiyuus (viz., the members of Sphere) in its "characters" section, so I put the pics together on a page.
For some reason, I had trouble keeping straight who was playing whom, and this page helped to fix it in my mind. Click here or on the image to go to full page:
There is also a nice shot of the four ladies together, with the two who seem to be the leads in this show in the foreground:
I enjoyed the first episode, with its not-yet-fully-explained emotions and apparently complex personalities and relationships. Not mere slice of life. It may take 2-3 episodes to understand exactly what it is.
I was happy that the somewhat less prominent two members of Sphere were the more central characters: Takagaki Ayahi and Kotobuki Minako. I think Ayahi is both the best actress and the best singer of the four. Bu I enjoyed them all in this show: Mina-chan as the genki and passionate sportswoman; Ayahi as an emotional and complex blonde; Tomacchan with a harsh and energetic voice; and Ainama as a soft-spoken miko.