She is the slime, she was the Demon King from levels ~11-20 in the form of the black slime. The blond fake saint lady is a grown up version of a child the slime ate.
I think the name change sometimes applies to Korean TL as well. It probably doesn't happen often due to some comics having en webtoon translations, but I've seen some translators change the name. Also machine translated Korean can work, but it makes all the characters feel shallow because google...