>>84606391She always had it as her planned backstory in the beginning, but I think once the Cerulean Cup = Tam fanclub plot took off, she just played along.
After Tam left, Cece, a wandering knight with definitely no ulterior motives, seeked her out and crowned her princess because she has a "crown" on her default outfit. Raora, desperate for any claim to Queen, "remembered" that she is actually Bijou's mother.
In addition to this, Bijou had a whole hat gacha arc where she spent tens of thousands of gold getting fucked until she got a Stain hat.
People started leaking to Gonathan that Bijou was turning evil so Gonathan suspected that it was the hat corrupting her and I think Bijou just played along with it and added the hat as a corrupting force, but it's more about the hat giving her her memories back as the Stain princess and this is her true nature.
And no, Raora does not know she signed up to be the Queen of the Stains lol.
