Renais was made to be ruled by a king and queen. These words had been drilled into their heads since childhood, and they understood the meaning behind them; Ephraim would grow up and marry, take Tana or the princess of a further off kingdom to be his wife, and then he would rule. Eirika would be traded like a bargaining chip to some other land, to strengthen the bonds between the two countries.
After that, they would see each other once every few years, if at all.
And so they treasured their childhood. They sparred with each other, and learned their letters leaning over the same enormous book. They shared pastries stolen from the kitchen, they shared gossip overheard from kitchen maids. They shared a best friend, and then they killed him together. They shared the burden of that too.
Finally, they were grown, but somehow it seemed like the world had just become younger. Behind almost every country now was a young lord like them, only just barely no longer a child, with too many lines around their eyes for someone their age. There were no adults to make the rules anymore, and their bonds were already strong between them; it didn't take a convenient marriage to trust L'arachel or Innes or Joshua. They would trust them with their lives, and would do the same with their countries.
And so Eirika stayed, and became, if possible, even more beloved to the people. Ephraim began to realize how cruel it would have been to tear her from this land. He, too, was loved, even if he did not realize it. Side-by-side they ruled, for their tutors had been right; Renais was meant to be ruled by a king and queen.