I don't believe it is explained in the manga, but it will be later down.
Spoilers
It's an imposter actually. The name is Joanna (per fan translation) who was also "royalty" because of black hair and also a saintess from another country. They have black hair because they used Emma's japanese face (Minato's) before she died. It's actually a gag on how awkward it is that a 30-yr old is acting as a kid in their academy. Thankfully Minato's japanese face is naturally young-looking so other people are not so weirded out. However, George and William who both knew how old the body was is holding their laughter and goosebumps.
As for the reason how they got face the magicians from the enemy country also has no idea (im pretty sure i will remember the reason if it was really mentioned bit i guess i forgot, so feel free to correct me).
Anyway, this fake saintess is actually an old man inside and has brainwashing power, or more like, power of suggestion. However, remember those hundreds of pins Emma made?, those have naturally dispelling power and remember that she has giving those away since the beginning? That's why only a few students were affected. And even those affected are easily cured.
Ohhhhhh!!my g! This girl, I mean man is ... let's say cosplaying using Minato's face but why they didn't react the first time he appeared? Also, what's his purpose going there? Is this a different country than that Japanese guy?
Spoiler
They didn't react because they didn't actually see "her". They heard of what happened but didn't really see her. Emma, George and William saw "her" in the school and Emma panicked. Like very badly. So did George and William coz they were so confused how Minato's body/face is in that world where they reincarnated. Thankfully Joshua was there and helped all three siblings to get home. Joshua carried Emma to the carriage. This actually made me bias to Joshua being the love interest (if there is any lol). But Joshua had no idea why they were panicking especially Emma who practically broke down.
Spoiler
Yes, it's a different country from the Imperial Japanese (or japanese version in that world). I believe it's called holy kingdom/holy empire or something but they are the ones who announces who the saintess is. But Emma became the unofficial "saintess" because of how pretty and kind she is. Remember that asked for the slum area as her reward and people saw that as a kind gesture. But it's a gag actually since she thought people are calling her "horny" not "holy" i guess it doesn't translate well in english but iirc the words for holy woman/saintess and horny girl is somehow close hence Emma misunderstood LOL
Oh? I thought Emma saw her on the day the Japanese guy was introduced. I mean the woman who walked with Emperor when they are doing their entrance, that's the woman I'm talking to. Is she different from the woman who appeared in the party where the siblings attended?
Ohhh sorry2 i misunderstood yeah that's a different person. I thought you meant the other "royal child" that was teased on the early chapters. I feel bad for the King because he was really loyal to his 2 wives. 1 queen, and 1 consort. So they think the impostor is a distant relative or something.
FYI - Rose is consort and the queen is actually really really good. Rose and Victoria (of course it's Victoria) are actually not hostile and are friends. Rose looks up to Victoria. And Victoria looks after Rose like an older sister.
Ohhh, now I see. So the woman who arrived with the emperor that time on the day Japanese guy appeared, I think she also have a black hair, Is that Victoria? Emma thought she's the royal consort at that time I think.
...I can't seem to remember if it was explained? You know that woman who appeared with black hair in the party (first party they met the King)? Is it the same girl in a different party where the Japanese guy attended? MC thought she was a consort when the King and that girl appeared.