@roeva it is Kurasaki Aya. It even says on the title pic.
イラスト:倉崎あや illustration : Kurasaki Aya
According to the Yatougo home page - http://yatougo179.web.fc2.com/ Yatougo is not the artist, he/she is the manager of the circle.
Yatougo's Pixiv - http://www.pixiv.net/member_illust.php?id=10223550 Quality artwork right there.
Also, the portrait icons with orange borders seems to characters from other Android apps (by various artists) https://play.google.com/store/apps/developer?id=%E3%83%A4%E3%83%88%E3%82%A6%E3%82%B4&hl=en