Serensiel is an elf woman Yuvenciel fell in love with 500 years ago. Their destiny was to become the Queen and the guardian (I’m guessing a similar role as Ritto) respectively.
Yuvenciel did not want this fate forced upon him and tried to persuade Serensiel from becoming the Queen and running away with him, to rebel against her Telezia basically. Yuvenciel claimed he loved her “with passion like a human”.
It didn’t work, Serenciel rejected him and became the Queen. Yuvenciel ran away and at some point became an Apostle. He wanted to freed elves from Telezia so no one would suffer the same fate as Serenciel.