>>54901701The character designs are drawn up first and they audition typically while the model is still somewhere in the production pipeline. While the talent doesn't get to change the design fundamentally (because they designs are picked for marketing reasons) they can offer some input before the model is finalized.
Bijou being a fan of Houseki no Kuni is probably just a coincidence, though there's a real possibility she picked up the series entirely because she got the rock model not the other way around.
Rissa probably auditioned for the bird one specifically so that she could be with her oshi. And as a lesbian idolfag she is highly qualified for the position.
