The birthmark is the mark of the curse casted by the first Emil. Notice the mark fading in the current emil's face when the forest burn and the demon lost his power
54 onwards shows a particular side of his cheek. At the end, the mark is gone. It may be because the curse is gone or the curse continues and new Emil is born again.
Uwu I'm so happy it got translated till the end. Thank you very Much Translators!!! Wish there was more sweet moments between komomo and natsu. They're so cute. I really love Komomo sama especially.
I think chapter 23.4 is not in proper order. Its very sweet though
They're finally acknowledging each other as their significant other.