Elysa is a Floette Life Domain Cleric of Xerneas and an inhabitant of the Fae Realm, a land that is in a state of perpetual change. Death can not exist in the Fae realm because it is a land of pure life energy, but what happens there may be weirder than death. Rather than death, transformation occurs, turning you from one thing to another till you no longer even have a sense of self. You might become a Tree, or a Living Rock, or warp into a bizarre creature, hence why the fae are often so flippant towards harming the lives of mortals. They lack the ability to comprehend death.

Elysa however learned of death, it was alien to her and very disturbing but she is trying to understand how mortals view life, and believes that life is worth preserving as long as she can keep it, but not just to keep people alive but so that they can also enjoy life. Life means nothing if you are being preserved as a vegetable, Life is meant to be lived freely and allowed to grow and change.

So yeah she is the reason Yueyue escaped her village and is now on the run with her.