Shotoku is familiar if you're into Touhou.
Miko = Prince Shotoku, who under Empress Suiko reformed the court. He built the temple seen here, Horyuji.
He sent Imoko on a mission trip to China that opened China-Japan relations and changed Japan into a power.
I have no idea who Tsukasa is meant to be, but since she's wearing what looks like a Chinese dress it could be Pei Shiqing, a Chinese diplomat who came to Japan with Imoko and later returned to China with Miko's correspondence.