they prolly plan to make him the ML
From my understanding, this is most likely a parallel universe with everything the same except for one difference, Rayvan are completely different ages.
Her first life, she raised him as he was a child abandoned by the emperor.
Her second life, Rayvan has no recollection of the FL so he grew up abandoned still, fought in the war and killed his family to take over the throne.
So it's not an icky story. However, on the flip side, due to the shock of seeing the FL die in front of him, awakened some sort of power where he manipulated reality and time, sent her soul back and he gained an older form, perhaps he was cursed to look like a child, at the cost of his memories of the FL.
Why is he a kid in her past life but an adult/teen in her second life （￣へ￣）