The Tale of an Eternal Era

The white nation, Albina. The royal couple has passed away, and the princess is only sixteen years old. The beautiful Io, who is always by her side, is her support. However, Io, a female knight, departs for the battlefield. While waiting for her return, the princess becomes aware of her romantic feelings and grows in her awareness as one who rules the nation. (Since it is a fictional kingdom, it is set in a different world, but there are no elements such as magic.)