
So here's the full storyline for those who are confused like me. Uke (15y/o) is forced by his newly found father to dress up as woman and become empress(to end emperor's lineage). Like the chp shown, the emperor is furious but after spending times together they actually grown closer. But when emperor's brother died the father changed his plan to giving uke a pills to able have child. So he had two childrens. And for 15 years seme never bother to meet him nor his childrens (but based what I see from the chp 0, I think they change uke's age to 20 because the time skip become 10 years, which is a good thing). Uke also changed, from weak type to having a more strong personality.
That's all I know from naver blog

At the age of fifteen, Mokdan struggled to survive under the care of his impoverished mother. However, his life took a turn when he unexpectedly reunited with his estranged father, the Prime Minister of the Emperor. His father offered him wealth and fame on the condition that Mokdan dressed as a woman and lived in the palace. The father's hidden motive was to establish a male Empress in the palace, thereby ending the royal family's lineage. Despite the difficult circumstances, Mokdan accepted the role of Empress, primarily out of concern for his mother. Over time, Mokdan's actions endeared him to the Emperor, who was moved by Mokdan's bravery when he foiled a plot to poison the Emperor. Gradually, the Emperor began to open his heart to Mokdan. Meanwhile, the Emperor's younger brother, who aimed to seize the throne, met an accidental death, leaving the Prime Minister with a new scheme. This plan involved giving Mokdan a medicine that could enable a man to conceive. To everyone's surprise, Mokdan miraculously gave birth to two children. Despite the newfound family, Mokdan grew weary of the distant Emperor, who had not been present for fifteen years, and the children felt isolated in the palace. One night, Mokdan decided to venture into the streets for a secret outing.

Read the ending (the last extra chapter) and it's confusing at first, but with the help of a comment i able to understand it. And it's depressing as hell.
Basically the seme accept his fate and become Sura, and the uke also accept his fate and stay with seme when the seme asked him if he is willing to live for him(seme). But at the last extra chapter it is implied that the uke doesn't love seme anymore and he is chained with the ring sewed in his finger (meaning he doesn't want to wear the ring seme give him anymore) and part of the reason why uke doesn't love him anymore is because seme already become "a god" and nobody able to love god, also the uke regain his memory so it could also be one of the reason (even though he already regain his memory in the last chapter he still wear the ring and accept his fate (live for the seme)).
Heejae you son of bitch, I approve your move