Shirokuro Tsukeru?

There are two monarchies in the celestial realm, each with their own rulers. Despite the ongoing war between them, Prince Sakuya of the Black Kingdom has formed a bond with Prince Rui and his sister Mio from the White Kingdom. They often visit a lake within neutral territory. One day, Mio frantically rushes to their meeting spot, but before she can speak, she is sucked into a strange portal and disappears into the darkness. Sakuya and Rui remain captivated by the location. Three years pass, and a girl dressed in foreign attire who bears an uncanny resemblance to Mio falls from the sky. However, she claims to be someone entirely different from the lower realm!