What were the events that led to this? (I'm wholly unfamiliar with the source material)
I'm not sure if this fanartist might have their own ongoing story between these three, but Raiden Ei is known to disappear into a pocket dimension and let her puppet rule the country in her place for years at a time in the game.
Ei became depressed after a massive disaster killed her twin sister and most of friends five hundred years before the start of the game. She created a puppet to rule in her place while she hid in her pocket dimension. She has only begun interacting with the world again after the protagonist defeated her.
