I thought that she also stayed because of a literal life-or-death situation (as she wasn't the original in 8-ish years she would die due to magic exposure without him as a counter)? Was this just a forgotten plot point? Or did I just misremember...
@mdescova She has to stay near the emperor because his country is a magic hazard for her.
They only discovered she's sensitive to magic because they went to his country that is abundant with it. Meanwhile, there's barely any in her kingdom that's why she didn't know about it before. So the emperor thought she's better of staying there