After some time the emperor starts to feel some admiration to Reirin in Keigetsu's body. He has a guilty concience because his only love is Reirin (I may dislike the man, but I respect his loyalty), so giving some basic respect to Keigetsu (the girl that swaped bodies) is a nono for him.
Anyways, the empress mother gets sick and Reirin needed a thing to cure her, she begs the emperor to lend her that thing, but he refuses, she says:
"Why?! Still doubting me? How can you believe me?!"
He was going to accept (by this time she already proveed herself to be trustworty), but then he tought that, by lending her that thing, he would "forgive her" and the only girl in his heart is Reirin, so trusting "Keigetsu" would be betraying Reirin, so he says:
"The day I trust you will never come."
The MC gets dissapointed and she says that he haven't heard her the 2 times before and this, and its fine, she will do it alone. When she runs to get the thing the emperor sees the reflection of Reirin and discovered that both have changed bodies so the one he treated bad was not Keigetsu, but Reirin.
After some stuff (that I am not going to spoil) happens, the emperor finds Reirin, he hugs her and he starts to apologize, when Reirin doesn't say anything lightning began to fall (the emperor has powers), then he says that if Reirin doesn't forgive him he will cut Keigetsu in eight pieces, inmediatly she says she will forgive him.
Take note, she forgives him not because of him, but because of Keigetsu, she really likes Keigetsu and sees her as a friend, while she only sees the emperor as her cousin, there was no mention of her feeling romantic love for him in all the novel.
After forcing her to accept his apologies, he still wants to harm Keigetsu so Reirin tells him that she have forgiven him only in words, that she is still sad, and if he really wants her to accept his apology he will pardon Keigetsu, after more talk the emperor wants Reirin to call him "cousin" like before (she called him "His Highness") and she accepts with one codition, she and Keigetsu will swap bodies time to time, if he can identify who is who she will call him "cousin" and will swear to remain being his fiancee.
The ending is Reirin talking to her servant that she needs to act better as Keigetsu because she doesn't want to get caught. Keigetsu accepted being her replacement and Reirin admits she love replacing Keigetsu, so this makes them best friends (this one was a little difficult to understand, by how I read it, Reirin thinks that sharing this secret makes them best friends, I could be wrong). And it ends with her servant telling her she really is a bad girl.
